Segment Reporting (Details) $ in Thousands	3 Months Ended		6 Months Ended
Segment Reporting (Details) $ in Thousands	Jun. 30, 2025 USD ($)	Jun. 30, 2024 USD ($)	Jun. 30, 2025 USD ($) Segment	Jun. 30, 2024 USD ($) Segment	Dec. 31, 2024 USD ($)
Segment Reporting [Abstract]
Number of reportable segments \| Segment			2	1
Financial Information of Reportable Segments [Abstract]
Net interest income	$ 124,220	$ 97,174	$ 231,443	$ 192,348
Provision for loan losses	17,835	8,899	25,389	14,478
Net interest income after provision for loan losses	106,385	88,275	206,054	177,870
Noninterest income [Abstract]
Service charges on deposit accounts	4,578	4,219	8,821	8,336
Card services income	6,077	5,587	11,394	10,782
Retirement plan administration fees	15,710	14,798	31,568	29,085
Wealth management	10,678	10,173	21,624	19,870
Insurance services	4,097	3,848	8,858	8,236
Bank owned life insurance income	2,180	1,834	5,577	4,186
Net securities gains (losses)	112	(92)	8	2,091
Other	3,500	2,865	6,534	6,038
Total noninterest income	46,932	43,232	94,384	88,624
Noninterest expense [Abstract]
Salaries and employee benefits	64,155	55,393	124,849	111,097
Technology and data services	10,804	9,249	21,042	18,999
Occupancy	9,038	7,671	18,065	15,769
Professional fees and outside services	5,021	4,565	9,973	9,418
Office supplies and postage	1,871	1,804	3,813	3,669
FDIC assessment	1,820	1,667	3,514	3,402
Advertising	974	873	2,112	1,685
Amortization of intangible assets	3,042	2,133	5,153	4,301
Loan collection and other real estate owned, net	489	715	1,148	1,268
Acquisition expenses	17,180	0	18,401	0
Other	8,216	5,518	14,440	11,753
Total noninterest expense	122,610	89,588	222,510	181,361
Income before income tax expense	30,707	41,919	77,928	85,133
Income tax expense	8,197	9,203	18,673	18,594
Net income	22,510	32,716	59,255	66,539
Goodwill	454,072	361,851	454,072	361,851	$ 362,663
Intangible assets, net	64,447	36,835	64,447	36,835	36,360
Total assets	16,014,781	13,501,909	$ 16,014,781	13,501,909	$ 13,786,666
Segment reporting, other segment item, composition, description			Included in All Other is the revenue and expenses from certain other non-bank subsidiaries of the parent, including the insurance subsidiary, along with eliminating amounts that do not meet the definition of an operating segment.
All Other [Member]
Financial Information of Reportable Segments [Abstract]
Net interest income	0	0	$ 0	0
Provision for loan losses	0	0	0	0
Net interest income after provision for loan losses	0	0	0	0
Noninterest income [Abstract]
Service charges on deposit accounts	0	0	0	0
Card services income	0	0	0	0
Retirement plan administration fees	(371)	(353)	(769)	(722)
Wealth management	8	13	19	26
Insurance services	4,096	3,848	8,857	8,235
Bank owned life insurance income	0	0	0	0
Net securities gains (losses)	0	0	0	0
Other	(1,663)	(1,919)	(4,397)	(4,685)
Total noninterest income	2,070	1,589	3,710	2,854
Noninterest expense [Abstract]
Salaries and employee benefits	2,796	2,677	5,488	5,363
Technology and data services	160	178	327	346
Occupancy	67	59	134	116
Professional fees and outside services	(351)	(326)	(718)	(666)
Office supplies and postage	15	13	29	28
FDIC assessment	0	0	0	0
Advertising	1	2	2	3
Amortization of intangible assets	57	16	116	34
Loan collection and other real estate owned, net	0	0	0	0
Acquisition expenses	0	0	0	0
Other	(1,742)	(1,882)	(4,292)	(4,451)
Total noninterest expense	1,003	737	1,086	773
Income before income tax expense	1,067	852	2,624	2,081
Income tax expense	0	41	0	167
Net income	1,067	811	2,624	1,914
Goodwill	14,536	14,377	14,536	14,377
Intangible assets, net	1,158	323	1,158	323
Total assets	(2,047,258)	(1,673,919)	(2,047,258)	(1,673,919)
Operating Segments [Member] \| Banking [Member]
Financial Information of Reportable Segments [Abstract]
Net interest income	124,202	97,156	231,407	192,313
Provision for loan losses	17,835	8,899	25,389	14,478
Net interest income after provision for loan losses	106,367	88,257	206,018	177,835
Noninterest income [Abstract]
Service charges on deposit accounts	4,578	4,219	8,821	8,336
Card services income	6,077	5,587	11,394	10,782
Retirement plan administration fees	0	0	0	0
Wealth management	10,153	9,660	20,490	18,847
Insurance services	1	0	1	1
Bank owned life insurance income	2,180	1,834	5,577	4,186
Net securities gains (losses)	112	(92)	8	2,091
Other	5,018	4,652	10,631	10,458
Total noninterest income	28,119	25,860	56,922	54,701
Noninterest expense [Abstract]
Salaries and employee benefits	52,659	44,349	102,207	89,474
Technology and data services	10,341	8,813	20,142	18,093
Occupancy	8,687	7,331	17,379	15,089
Professional fees and outside services	4,851	4,515	9,666	9,239
Office supplies and postage	1,783	1,710	3,664	3,468
FDIC assessment	1,820	1,667	3,514	3,402
Advertising	952	859	2,070	1,655
Amortization of intangible assets	2,500	1,666	4,008	3,348
Loan collection and other real estate owned, net	489	715	1,148	1,268
Acquisition expenses	17,180	0	18,401	0
Other	9,684	7,101	18,224	15,627
Total noninterest expense	110,946	78,726	200,423	160,663
Income before income tax expense	23,540	35,391	62,517	71,873
Income tax expense	6,917	7,947	15,968	16,001
Net income	16,623	27,444	46,549	55,872
Goodwill	415,659	324,250	415,659	324,250
Intangible assets, net	56,785	30,624	56,785	30,624
Total assets	18,013,397	15,132,919	18,013,397	15,132,919
Operating Segments [Member] \| Retirement Plan Administration [Member]
Financial Information of Reportable Segments [Abstract]
Net interest income	18	18	36	35
Provision for loan losses	0	0	0	0
Net interest income after provision for loan losses	18	18	36	35
Noninterest income [Abstract]
Service charges on deposit accounts	0	0	0	0
Card services income	0	0	0	0
Retirement plan administration fees	16,081	15,151	32,337	29,807
Wealth management	517	500	1,115	997
Insurance services	0	0	0	0
Bank owned life insurance income	0	0	0	0
Net securities gains (losses)	0	0	0	0
Other	145	132	300	265
Total noninterest income	16,743	15,783	33,752	31,069
Noninterest expense [Abstract]
Salaries and employee benefits	8,700	8,367	17,154	16,260
Technology and data services	303	258	573	560
Occupancy	284	281	552	564
Professional fees and outside services	521	376	1,025	845
Office supplies and postage	73	81	120	173
FDIC assessment	0	0	0	0
Advertising	21	12	40	27
Amortization of intangible assets	485	451	1,029	919
Loan collection and other real estate owned, net	0	0	0	0
Acquisition expenses	0	0	0	0
Other	274	299	508	577
Total noninterest expense	10,661	10,125	21,001	19,925
Income before income tax expense	6,100	5,676	12,787	11,179
Income tax expense	1,280	1,215	2,705	2,426
Net income	4,820	4,461	10,082	8,753
Goodwill	23,877	23,224	23,877	23,224
Intangible assets, net	6,504	5,888	6,504	5,888
Total assets	$ 48,642	$ 42,909	$ 48,642	$ 42,909

NBT BANCORP INC 10-Q