SEGMENT INFORMATION - Significant Segment Expenses and Measure of Segment Profit or Loss (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024	Dec. 31, 2024
Segment Reporting, Asset Reconciling Item [Line Items]
Net interest income (expense)	$ 1,734	$ 1,796	$ 5,092	$ 5,434
Rental income on operating lease equipment	273	262	815	776
All other noninterest income	426	388	1,197	1,140
Total noninterest income	699	650	2,012	1,916
Total revenue	2,433	2,446	7,104	7,350
Depreciation on operating lease equipment	98	99	296	293
Maintenance and other operating lease expenses	67	59	180	164
Personnel cost	817	788	2,445	2,277
Acquisition-related expenses	28	46	108	148
All other noninterest expense	481	464	1,455	1,336
Total noninterest expense	1,491	1,456	4,484	4,218
Provision for credit losses	191	117	460	276
Income before income taxes	751	873	2,160	2,856
Income tax expense	183	234	534	779
Net income	568	639	1,626	2,077
Loans and leases	144,758	138,695	144,758	138,695	$ 140,221
Operating lease equipment, net	9,446	9,186	9,446	9,186	9,323
Deposits	163,190	151,574	163,190	151,574	$ 155,229
Operating Segments \| General Bank
Segment Reporting, Asset Reconciling Item [Line Items]
Net interest income (expense)	846	760	2,458	2,174
Rental income on operating lease equipment	0	0	0	0
All other noninterest income	166	149	494	446
Total noninterest income	166	149	494	446
Total revenue	1,012	909	2,952	2,620
Depreciation on operating lease equipment	0	0	0	0
Maintenance and other operating lease expenses	0	0	0	0
Personnel cost	213	212	637	604
Acquisition-related expenses	0	0	0	0
All other noninterest expense	369	341	1,090	978
Total noninterest expense	582	553	1,727	1,582
Provision for credit losses	1	55	60	113
Income before income taxes	429	301	1,165	925
Income tax expense	109	99	298	270
Net income	320	202	867	655
Loans and leases	65,225	64,254	65,225	64,254
Operating lease equipment, net	0	0	0	0
Deposits	74,596	71,898	74,596	71,898
Operating Segments \| Commercial Bank
Segment Reporting, Asset Reconciling Item [Line Items]
Net interest income (expense)	303	305	895	916
Rental income on operating lease equipment	54	57	164	172
All other noninterest income	101	79	268	239
Total noninterest income	155	136	432	411
Total revenue	458	441	1,327	1,327
Depreciation on operating lease equipment	43	47	131	141
Maintenance and other operating lease expenses	0	0	0	0
Personnel cost	74	68	215	205
Acquisition-related expenses	0	0	0	0
All other noninterest expense	149	150	462	425
Total noninterest expense	266	265	808	771
Provision for credit losses	168	11	300	70
Income before income taxes	24	165	219	486
Income tax expense	6	41	56	127
Net income	18	124	163	359
Loans and leases	38,841	37,281	38,841	37,281
Operating lease equipment, net	737	767	737	767
Deposits	2,978	3,126	2,978	3,126
Operating Segments \| SVB Commercial
Segment Reporting, Asset Reconciling Item [Line Items]
Net interest income (expense)	493	560	1,476	1,636
Rental income on operating lease equipment	0	0	0	0
All other noninterest income	135	137	397	405
Total noninterest income	135	137	397	405
Total revenue	628	697	1,873	2,041
Depreciation on operating lease equipment	0	0	0	0
Maintenance and other operating lease expenses	0	0	0	0
Personnel cost	106	112	330	354
Acquisition-related expenses	0	0	0	0
All other noninterest expense	267	272	804	765
Total noninterest expense	373	384	1,134	1,119
Provision for credit losses	22	51	100	93
Income before income taxes	233	262	639	829
Income tax expense	58	75	162	235
Net income	175	187	477	594
Loans and leases	40,629	37,098	40,629	37,098
Operating lease equipment, net	0	0	0	0
Deposits	39,891	35,844	39,891	35,844
Operating Segments \| Rail
Segment Reporting, Asset Reconciling Item [Line Items]
Net interest income (expense)	(55)	(48)	(160)	(136)
Rental income on operating lease equipment	219	205	651	604
All other noninterest income	2	2	7	8
Total noninterest income	221	207	658	612
Total revenue	166	159	498	476
Depreciation on operating lease equipment	55	52	165	152
Maintenance and other operating lease expenses	67	59	180	164
Personnel cost	6	6	20	20
Acquisition-related expenses	0	0	0	0
All other noninterest expense	16	14	56	43
Total noninterest expense	144	131	421	379
Provision for credit losses	0	0	0	0
Income before income taxes	22	28	77	97
Income tax expense	5	8	19	27
Net income	17	20	58	70
Loans and leases	63	62	63	62
Operating lease equipment, net	8,709	8,419	8,709	8,419
Deposits	2	14	2	14
Corporate
Segment Reporting, Asset Reconciling Item [Line Items]
Net interest income (expense)	147	219	423	844
Rental income on operating lease equipment	0	0	0	0
All other noninterest income	22	21	31	42
Total noninterest income	22	21	31	42
Total revenue	169	240	454	886
Depreciation on operating lease equipment	0	0	0	0
Maintenance and other operating lease expenses	0	0	0	0
Personnel cost	418	390	1,243	1,094
Acquisition-related expenses	28	46	108	148
All other noninterest expense	(320)	(313)	(957)	(875)
Total noninterest expense	126	123	394	367
Provision for credit losses	0	0	0	0
Income before income taxes	43	117	60	519
Income tax expense	5	11	(1)	120
Net income	38	106	61	399
Loans and leases	0	0	0	0
Operating lease equipment, net	0	0	0	0
Deposits	$ 45,723	$ 40,692	$ 45,723	$ 40,692

FIRST CITIZENS BANCSHARES INC /DE/ 10-Q