LOANS AND ALLOWANCE FOR CREDIT LOSSES - Amount of Business Lending Loans by Credit Quality Categories (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2026	Dec. 31, 2025
LOANS AND ALLOWANCE FOR CREDIT LOSSES
Total Loans	$ 11,131,184	$ 10,949,757
Legacy Loan
LOANS AND ALLOWANCE FOR CREDIT LOSSES
Total Loans	11,131,184	10,949,757
Business lending
LOANS AND ALLOWANCE FOR CREDIT LOSSES
Year One	198,353	696,759
Year Two	655,146	340,994
Year Three	331,740	308,296
Year Four	283,797	503,612
Year Five	487,317	251,416
Prior	1,014,465	817,157
Revolving Loans Amortized Cost Basis	1,136,716	1,034,834
Revolving Loans Converted to Term	775,917	780,799
Total Loans	4,883,451	4,733,867
Year One writeoff	0	0
Year Two writeoff	0	0
Year Three writeoff	0	235
Year Four writeoff	134	256
Year Five writeoff	0	1,280
Prior	0	141
Revolving Loans Amortized Cost Basis	212	731
Revolving Loans Converted to Term	9	4,491
Total Loans	355	7,134
Business lending \| Pass
LOANS AND ALLOWANCE FOR CREDIT LOSSES
Year One	197,768	685,535
Year Two	646,820	318,453
Year Three	313,327	277,253
Year Four	248,920	435,037
Year Five	424,109	230,593
Prior	931,224	757,072
Revolving Loans Amortized Cost Basis	1,061,795	944,412
Revolving Loans Converted to Term	687,705	670,384
Total Loans	4,511,668	4,318,739
Business lending \| Special Mention
LOANS AND ALLOWANCE FOR CREDIT LOSSES
Year One	585	6,552
Year Two	2,692	18,536
Year Three	12,046	17,641
Year Four	12,943	12,878
Year Five	9,635	12,954
Prior	41,390	30,586
Revolving Loans Amortized Cost Basis	38,647	59,539
Revolving Loans Converted to Term	59,239	85,726
Total Loans	177,177	244,412
Business lending \| Substandard
LOANS AND ALLOWANCE FOR CREDIT LOSSES
Year One	0	4,672
Year Two	5,634	4,005
Year Three	6,367	13,402
Year Four	21,934	55,697
Year Five	53,573	7,869
Prior	41,851	29,499
Revolving Loans Amortized Cost Basis	36,274	29,305
Revolving Loans Converted to Term	27,421	24,689
Total Loans	193,054	169,138
Business lending \| Doubtful
LOANS AND ALLOWANCE FOR CREDIT LOSSES
Year One	0	0
Year Two	0	0
Year Three	0	0
Year Four	0	0
Year Five	0	0
Prior	0	0
Revolving Loans Amortized Cost Basis	0	1,578
Revolving Loans Converted to Term	1,552	0
Total Loans	1,552	1,578
CRE - multifamily
LOANS AND ALLOWANCE FOR CREDIT LOSSES
Year One	16,467	138,662
Year Two	140,251	13,544
Year Three	12,742	99,356
Year Four	98,060	135,285
Year Five	132,520	48,348
Prior	186,254	143,633
Revolving Loans Amortized Cost Basis	136,794	137,755
Revolving Loans Converted to Term	197,137	201,003
Total Loans	920,225	917,586
Year One writeoff	0	0
Year Two writeoff	0	0
Year Three writeoff	0	0
Year Four writeoff	0	0
Year Five writeoff	0	19
Prior	0	82
Revolving Loans Amortized Cost Basis	0	428
Revolving Loans Converted to Term	0	0
Total Loans	0	529
CRE - multifamily \| Pass
LOANS AND ALLOWANCE FOR CREDIT LOSSES
Year One	16,467	138,662
Year Two	140,251	13,544
Year Three	12,742	90,834
Year Four	89,246	125,581
Year Five	132,520	38,245
Prior	163,213	130,435
Revolving Loans Amortized Cost Basis	135,197	131,129
Revolving Loans Converted to Term	182,618	173,234
Total Loans	872,254	841,664
CRE - multifamily \| Special Mention
LOANS AND ALLOWANCE FOR CREDIT LOSSES
Year One	0	0
Year Two	0	0
Year Three	0	8,522
Year Four	0	9,317
Year Five	0	9,019
Prior	9,105	5,089
Revolving Loans Amortized Cost Basis	0	4,979
Revolving Loans Converted to Term	11,600	24,825
Total Loans	20,705	61,751
CRE - multifamily \| Substandard
LOANS AND ALLOWANCE FOR CREDIT LOSSES
Year One	0	0
Year Two	0	0
Year Three	0	0
Year Four	8,814	387
Year Five	0	1,084
Prior	13,936	8,109
Revolving Loans Amortized Cost Basis	1,597	1,647
Revolving Loans Converted to Term	2,919	2,944
Total Loans	27,266	14,171
CRE - multifamily \| Doubtful
LOANS AND ALLOWANCE FOR CREDIT LOSSES
Year One	0	0
Year Two	0	0
Year Three	0	0
Year Four	0	0
Year Five	0	0
Prior	0	0
Revolving Loans Amortized Cost Basis	0	0
Revolving Loans Converted to Term	0	0
Total Loans	0	0
CRE - owner occupied
LOANS AND ALLOWANCE FOR CREDIT LOSSES
Year One	40,020	122,128
Year Two	103,053	95,357
Year Three	94,021	44,017
Year Four	39,906	68,293
Year Five	66,202	50,942
Prior	277,491	243,329
Revolving Loans Amortized Cost Basis	38,829	20,971
Revolving Loans Converted to Term	220,016	226,764
Total Loans	879,538	871,801
Year One writeoff	0	0
Year Two writeoff	0	0
Year Three writeoff	0	0
Year Four writeoff	134	47
Year Five writeoff	0	0
Prior	0	9
Revolving Loans Amortized Cost Basis	0	0
Revolving Loans Converted to Term	0	0
Total Loans	134	56
CRE - owner occupied \| Pass
LOANS AND ALLOWANCE FOR CREDIT LOSSES
Year One	40,020	119,369
Year Two	100,179	87,429
Year Three	88,843	39,911
Year Four	35,991	60,476
Year Five	57,074	47,869
Prior	261,901	228,439
Revolving Loans Amortized Cost Basis	37,438	20,295
Revolving Loans Converted to Term	201,080	192,919
Total Loans	822,526	796,707
CRE - owner occupied \| Special Mention
LOANS AND ALLOWANCE FOR CREDIT LOSSES
Year One	0	1,310
Year Two	1,303	6,357
Year Three	3,617	3,082
Year Four	3,025	751
Year Five	3,817	0
Prior	4,608	4,995
Revolving Loans Amortized Cost Basis	1,061	339
Revolving Loans Converted to Term	16,404	28,113
Total Loans	33,835	44,947
CRE - owner occupied \| Substandard
LOANS AND ALLOWANCE FOR CREDIT LOSSES
Year One	0	1,449
Year Two	1,571	1,571
Year Three	1,561	1,024
Year Four	890	7,066
Year Five	5,311	3,073
Prior	10,982	9,895
Revolving Loans Amortized Cost Basis	330	337
Revolving Loans Converted to Term	2,532	5,732
Total Loans	23,177	30,147
CRE - owner occupied \| Doubtful
LOANS AND ALLOWANCE FOR CREDIT LOSSES
Year One	0	0
Year Two	0	0
Year Three	0	0
Year Four	0	0
Year Five	0	0
Prior	0	0
Revolving Loans Amortized Cost Basis	0	0
Revolving Loans Converted to Term	0	0
Total Loans	0	0
CRE - non-owner occupied
LOANS AND ALLOWANCE FOR CREDIT LOSSES
Year One	91,425	185,602
Year Two	181,304	69,062
Year Three	68,603	110,535
Year Four	94,947	234,431
Year Five	231,589	101,392
Prior	420,132	337,667
Revolving Loans Amortized Cost Basis	448,683	376,817
Revolving Loans Converted to Term	250,207	254,945
Total Loans	1,786,890	1,670,451
Year One writeoff	0	0
Year Two writeoff	0	0
Year Three writeoff	0	0
Year Four writeoff	0	0
Year Five writeoff	0	1,111
Prior	0	0
Revolving Loans Amortized Cost Basis	0	0
Revolving Loans Converted to Term	0	3,198
Total Loans	0	4,309
CRE - non-owner occupied \| Pass
LOANS AND ALLOWANCE FOR CREDIT LOSSES
Year One	91,425	182,535
Year Two	181,304	67,569
Year Three	67,120	97,829
Year Four	78,557	189,423
Year Five	184,705	97,178
Prior	384,200	311,706
Revolving Loans Amortized Cost Basis	423,986	351,501
Revolving Loans Converted to Term	209,661	218,249
Total Loans	1,620,958	1,515,990
CRE - non-owner occupied \| Special Mention
LOANS AND ALLOWANCE FOR CREDIT LOSSES
Year One	0	3,067
Year Two	0	1,407
Year Three	1,401	4,748
Year Four	8,472	0
Year Five	2,105	2,593
Prior	25,713	19,642
Revolving Loans Amortized Cost Basis	18,265	19,024
Revolving Loans Converted to Term	24,492	24,679
Total Loans	80,448	75,160
CRE - non-owner occupied \| Substandard
LOANS AND ALLOWANCE FOR CREDIT LOSSES
Year One	0	0
Year Two	0	86
Year Three	82	7,958
Year Four	7,918	45,008
Year Five	44,779	1,621
Prior	10,219	6,319
Revolving Loans Amortized Cost Basis	6,432	6,292
Revolving Loans Converted to Term	16,054	12,017
Total Loans	85,484	79,301
CRE - non-owner occupied \| Doubtful
LOANS AND ALLOWANCE FOR CREDIT LOSSES
Year One	0	0
Year Two	0	0
Year Three	0	0
Year Four	0	0
Year Five	0	0
Prior	0	0
Revolving Loans Amortized Cost Basis	0	0
Revolving Loans Converted to Term	0	0
Total Loans	0	0
Commercial & industrial and other business loans
LOANS AND ALLOWANCE FOR CREDIT LOSSES
Year One	50,441	250,367
Year Two	230,538	163,031
Year Three	156,374	54,388
Year Four	50,884	65,603
Year Five	57,006	50,734
Prior	130,588	92,528
Revolving Loans Amortized Cost Basis	512,410	499,291
Revolving Loans Converted to Term	108,557	98,087
Total Loans	1,296,798	1,274,029
Year One writeoff	0	0
Year Two writeoff	0	0
Year Three writeoff	0	235
Year Four writeoff	0	209
Year Five writeoff	0	150
Prior	0	50
Revolving Loans Amortized Cost Basis	212	303
Revolving Loans Converted to Term	9	1,293
Total Loans	221	2,240
Commercial & industrial and other business loans \| Pass
LOANS AND ALLOWANCE FOR CREDIT LOSSES
Year One	49,856	244,969
Year Two	225,086	149,911
Year Three	144,622	48,679
Year Four	45,126	59,557
Year Five	49,810	47,301
Prior	121,910	86,492
Revolving Loans Amortized Cost Basis	465,174	441,487
Revolving Loans Converted to Term	94,346	85,982
Total Loans	1,195,930	1,164,378
Commercial & industrial and other business loans \| Special Mention
LOANS AND ALLOWANCE FOR CREDIT LOSSES
Year One	585	2,175
Year Two	1,389	10,772
Year Three	7,028	1,289
Year Four	1,446	2,810
Year Five	3,713	1,342
Prior	1,964	860
Revolving Loans Amortized Cost Basis	19,321	35,197
Revolving Loans Converted to Term	6,743	8,109
Total Loans	42,189	62,554
Commercial & industrial and other business loans \| Substandard
LOANS AND ALLOWANCE FOR CREDIT LOSSES
Year One	0	3,223
Year Two	4,063	2,348
Year Three	4,724	4,420
Year Four	4,312	3,236
Year Five	3,483	2,091
Prior	6,714	5,176
Revolving Loans Amortized Cost Basis	27,915	21,029
Revolving Loans Converted to Term	5,916	3,996
Total Loans	57,127	45,519
Commercial & industrial and other business loans \| Doubtful
LOANS AND ALLOWANCE FOR CREDIT LOSSES
Year One	0	0
Year Two	0	0
Year Three	0	0
Year Four	0	0
Year Five	0	0
Prior	0	0
Revolving Loans Amortized Cost Basis	0	1,578
Revolving Loans Converted to Term	1,552	0
Total Loans	1,552	1,578
Consumer mortgage
LOANS AND ALLOWANCE FOR CREDIT LOSSES
Year One	79,065	410,625
Year Two	402,400	425,234
Year Three	418,892	434,691
Year Four	424,951	429,944
Year Five	419,721	525,337
Prior	1,628,559	1,151,550
Revolving Loans Amortized Cost Basis	27,349	37,087
Revolving Loans Converted to Term	218,130	202,718
Total Loans	3,619,067	3,617,186
Year One writeoff	0	0
Year Two writeoff	0	0
Year Three writeoff	0	21
Year Four writeoff	0	5
Year Five writeoff	0	0
Prior	49	30
Revolving Loans Amortized Cost Basis	0	0
Revolving Loans Converted to Term	0	0
Total Loans	49	56
Consumer mortgage \| FICO AB
LOANS AND ALLOWANCE FOR CREDIT LOSSES
Year One	55,988	293,599
Year Two	275,261	279,861
Year Three	275,023	303,479
Year Four	296,198	299,020
Year Five	291,903	383,131
Prior	1,089,161	739,133
Revolving Loans Amortized Cost Basis	19,417	22,665
Revolving Loans Converted to Term	158,485	148,004
Total Loans	2,461,436	2,468,892
Consumer mortgage \| FICO AB \| Performing
LOANS AND ALLOWANCE FOR CREDIT LOSSES
Year One	55,988	293,599
Year Two	275,261	279,289
Year Three	274,243	302,498
Year Four	294,841	298,302
Year Five	291,032	382,256
Prior	1,085,295	736,261
Revolving Loans Amortized Cost Basis	19,417	22,665
Revolving Loans Converted to Term	158,293	148,004
Total Loans	2,454,370	2,462,874
Consumer mortgage \| FICO AB \| Nonperforming
LOANS AND ALLOWANCE FOR CREDIT LOSSES
Year One	0	0
Year Two	0	572
Year Three	780	981
Year Four	1,357	718
Year Five	871	875
Prior	3,866	2,872
Revolving Loans Amortized Cost Basis	0	0
Revolving Loans Converted to Term	192	0
Total Loans	7,066	6,018
Consumer mortgage \| FICO CDE
LOANS AND ALLOWANCE FOR CREDIT LOSSES
Year One	23,077	117,026
Year Two	127,139	145,373
Year Three	143,869	131,212
Year Four	128,753	130,924
Year Five	127,818	142,206
Prior	539,398	412,417
Revolving Loans Amortized Cost Basis	7,932	14,422
Revolving Loans Converted to Term	59,645	54,714
Total Loans	1,157,631	1,148,294
Consumer mortgage \| FICO CDE \| Performing
LOANS AND ALLOWANCE FOR CREDIT LOSSES
Year One	23,077	117,026
Year Two	127,139	141,528
Year Three	140,504	127,586
Year Four	125,091	126,599
Year Five	123,054	140,660
Prior	526,238	400,453
Revolving Loans Amortized Cost Basis	7,932	14,422
Revolving Loans Converted to Term	58,716	53,855
Total Loans	1,131,751	1,122,129
Consumer mortgage \| FICO CDE \| Nonperforming
LOANS AND ALLOWANCE FOR CREDIT LOSSES
Year One	0	0
Year Two	0	3,845
Year Three	3,365	3,626
Year Four	3,662	4,325
Year Five	4,764	1,546
Prior	13,160	11,964
Revolving Loans Amortized Cost Basis	0	0
Revolving Loans Converted to Term	929	859
Total Loans	25,880	26,165
Consumer indirect
LOANS AND ALLOWANCE FOR CREDIT LOSSES
Year One	222,839	760,600
Year Two	712,660	458,689
Year Three	410,191	312,492
Year Four	274,244	217,995
Year Five	185,443	69,888
Prior	88,634	39,690
Revolving Loans Amortized Cost Basis	0	0
Revolving Loans Converted to Term	0	0
Total Loans	1,894,011	1,859,354
Year One writeoff	0	1,199
Year Two writeoff	587	3,323
Year Three writeoff	985	4,127
Year Four writeoff	817	2,838
Year Five writeoff	619	1,135
Prior	449	948
Revolving Loans Amortized Cost Basis	0	0
Revolving Loans Converted to Term	0	0
Total Loans	3,457	13,570
Consumer indirect \| Performing
LOANS AND ALLOWANCE FOR CREDIT LOSSES
Year One	222,839	760,499
Year Two	712,495	458,410
Year Three	409,911	312,256
Year Four	274,063	217,772
Year Five	185,126	69,818
Prior	88,586	39,547
Revolving Loans Amortized Cost Basis	0	0
Revolving Loans Converted to Term	0	0
Total Loans	1,893,020	1,858,302
Consumer indirect \| Nonperforming
LOANS AND ALLOWANCE FOR CREDIT LOSSES
Year One	0	101
Year Two	165	279
Year Three	280	236
Year Four	181	223
Year Five	317	70
Prior	48	143
Revolving Loans Amortized Cost Basis	0	0
Revolving Loans Converted to Term	0	0
Total Loans	991	1,052
Consumer direct
LOANS AND ALLOWANCE FOR CREDIT LOSSES
Year One	25,696	88,937
Year Two	74,575	53,274
Year Three	46,154	29,551
Year Four	25,018	15,602
Year Five	12,375	4,908
Prior	9,126	5,897
Revolving Loans Amortized Cost Basis	7,209	7,348
Revolving Loans Converted to Term	63	78
Total Loans	200,216	205,595
Year One writeoff	0	205
Year Two writeoff	170	891
Year Three writeoff	345	701
Year Four writeoff	75	392
Year Five writeoff	90	55
Prior	304	25
Revolving Loans Amortized Cost Basis	1,251	232
Revolving Loans Converted to Term	0	0
Total Loans	2,235	2,501
Consumer direct \| Legacy Loan
LOANS AND ALLOWANCE FOR CREDIT LOSSES
Total Loans		205,595
Consumer direct \| Performing
LOANS AND ALLOWANCE FOR CREDIT LOSSES
Year One	25,696	88,898
Year Two	74,558	53,200
Year Three	46,111	29,486
Year Four	24,982	15,546
Year Five	12,330	4,904
Prior	9,068	5,846
Revolving Loans Amortized Cost Basis	7,135	7,251
Revolving Loans Converted to Term	63	71
Total Loans	199,943	205,202
Consumer direct \| Nonperforming
LOANS AND ALLOWANCE FOR CREDIT LOSSES
Year One	0	39
Year Two	17	74
Year Three	43	65
Year Four	36	56
Year Five	45	4
Prior	58	51
Revolving Loans Amortized Cost Basis	74	97
Revolving Loans Converted to Term	0	7
Total Loans	273	393
Home equity
LOANS AND ALLOWANCE FOR CREDIT LOSSES
Year One	9,193	65,113
Year Two	63,121	61,277
Year Three	59,026	45,172
Year Four	42,928	47,184
Year Five	45,247	45,576
Prior	106,920	66,856
Revolving Loans Amortized Cost Basis	179,176	173,402
Revolving Loans Converted to Term	28,828	29,175
Total Loans	534,439	533,755
Year One writeoff	0	0
Year Two writeoff	0	0
Year Three writeoff	0	112
Year Four writeoff	0	0
Year Five writeoff	0	0
Prior	0	34
Revolving Loans Amortized Cost Basis	9	7
Revolving Loans Converted to Term	0	0
Total Loans	9	153
Home equity \| Performing
LOANS AND ALLOWANCE FOR CREDIT LOSSES
Year One	9,193	65,113
Year Two	63,121	60,987
Year Three	58,435	44,399
Year Four	42,151	46,641
Year Five	44,730	45,505
Prior	106,479	66,152
Revolving Loans Amortized Cost Basis	178,452	172,782
Revolving Loans Converted to Term	28,748	29,091
Total Loans	531,309	530,670
Home equity \| Nonperforming
LOANS AND ALLOWANCE FOR CREDIT LOSSES
Year One	0	0
Year Two	0	290
Year Three	591	773
Year Four	777	543
Year Five	517	71
Prior	441	704
Revolving Loans Amortized Cost Basis	724	620
Revolving Loans Converted to Term	80	84
Total Loans	$ 3,130	$ 3,085

COMMUNITY FINANCIAL SYSTEM, INC. 10-Q