Business Segments - Results of Operations and Average Assets by Segment (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Segment Reporting Information
Net interest income	$ 1,384	$ 1,517
Provision for (benefit from) credit losses	94	164
Net Interest Income After Provision for Credit Losses	1,290	1,353
Noninterest Income
Wealth and asset management revenue	161	146
Service charges on deposits	151	137
Commercial banking revenue	143	161
Card and processing revenue	102	100
Mortgage banking net revenue	54	69
Leasing business revenue	39	57
Other noninterest income	50	22
Securities gains, net	10	4
Securities gains, net – non-qualifying hedges on MSRs	0	0
Total noninterest income	710	696
Noninterest Expense
Compensation and benefits	753	757
Technology and communications	117	118
Net occupancy expense	87	81
Equipment expense	37	37
Leasing business expense	32	29
Marketing expense	25	34
Card and processing expense	20	22
Other noninterest expense	(271)	(253)
Total noninterest expense	1,342	1,331
Income (loss) before income taxes	658	718
Applicable income tax expense (benefit)	138	160
Net Income	520	558
Total goodwill	4,918	4,915	$ 4,919
Total assets	214,506	208,657
Impairment losses on bank premises	0	1
Bank premises and equipment held for sale	16		19
Impairment losses and termination charges	0	1
Operating Segments | Commercial Banking
Segment Reporting Information
Net interest income	686	977
Provision for (benefit from) credit losses	71	46
Net Interest Income After Provision for Credit Losses	615	931
Noninterest Income
Wealth and asset management revenue	1	1
Service charges on deposits	100	87
Commercial banking revenue	142	160
Card and processing revenue	25	22
Mortgage banking net revenue	0	0
Leasing business revenue	39	57
Other noninterest income	18	16
Securities gains, net	1	(7)
Securities gains, net – non-qualifying hedges on MSRs	0	0
Total noninterest income	326	336
Noninterest Expense
Compensation and benefits	189	190
Technology and communications	4	2
Net occupancy expense	10	11
Equipment expense	7	7
Leasing business expense	1	0
Marketing expense	25	34
Card and processing expense	2	3
Other noninterest expense	(263)	(304)
Total noninterest expense	501	551
Income (loss) before income taxes	440	716
Applicable income tax expense (benefit)	71	136
Net Income	369	580
Total goodwill	2,324	2,324
Total assets	77,401	83,545
Operating Segments | Consumer and Small Business Banking
Segment Reporting Information
Net interest income	1,125	1,257
Provision for (benefit from) credit losses	84	51
Net Interest Income After Provision for Credit Losses	1,041	1,206
Noninterest Income
Wealth and asset management revenue	59	53
Service charges on deposits	51	51
Commercial banking revenue	1	1
Card and processing revenue	74	74
Mortgage banking net revenue	54	69
Leasing business revenue	0	0
Other noninterest income	27	25
Securities gains, net	0	0
Securities gains, net – non-qualifying hedges on MSRs	0	0
Total noninterest income	266	273
Noninterest Expense
Compensation and benefits	234	224
Technology and communications	7	7
Net occupancy expense	53	51
Equipment expense	13	11
Leasing business expense	22	17
Marketing expense	0	0
Card and processing expense	19	20
Other noninterest expense	(291)	(315)
Total noninterest expense	639	645
Income (loss) before income taxes	668	834
Applicable income tax expense (benefit)	141	175
Net Income	527	659
Total goodwill	2,369	2,365
Total assets	89,219	85,296
Impairment losses on bank premises		1
Operating Segments | Wealth and Asset Management
Segment Reporting Information
Net interest income	59	101
Provision for (benefit from) credit losses	0	0
Net Interest Income After Provision for Credit Losses	59	101
Noninterest Income
Wealth and asset management revenue	101	92
Service charges on deposits	0	0
Commercial banking revenue	0	0
Card and processing revenue	1	1
Mortgage banking net revenue	0	0
Leasing business revenue	0	0
Other noninterest income	0	(1)
Securities gains, net	0	0
Securities gains, net – non-qualifying hedges on MSRs	0	0
Total noninterest income	102	92
Noninterest Expense
Compensation and benefits	62	61
Technology and communications	0	0
Net occupancy expense	3	3
Equipment expense	0	0
Leasing business expense	0	0
Marketing expense	0	0
Card and processing expense	0	0
Other noninterest expense	(38)	(36)
Total noninterest expense	103	100
Income (loss) before income taxes	58	93
Applicable income tax expense (benefit)	12	19
Net Income	46	74
Total goodwill	225	226
Total assets	11,360	11,707
Operating Segments | Wealth and Asset Management | Revision of Prior Period, Adjustment
Noninterest Income
Wealth and asset management revenue			(186)	$ (177)	$ (180)
Noninterest Expense
Other noninterest expense			$ 186	$ 177	$ 180
General Corporate and Other
Segment Reporting Information
Net interest income	(486)	(818)
Provision for (benefit from) credit losses	(61)	67
Net Interest Income After Provision for Credit Losses	(425)	(885)
Noninterest Income
Wealth and asset management revenue	0	0
Service charges on deposits	0	(1)
Commercial banking revenue	0	0
Card and processing revenue	2	3
Mortgage banking net revenue	0	0
Leasing business revenue	0	0
Other noninterest income	5	(18)
Securities gains, net	9	11
Securities gains, net – non-qualifying hedges on MSRs	0	0
Total noninterest income	16	(5)
Noninterest Expense
Compensation and benefits	268	282
Technology and communications	106	109
Net occupancy expense	21	16
Equipment expense	17	19
Leasing business expense	9	12
Marketing expense	0	0
Card and processing expense	(1)	(1)
Other noninterest expense	321	402
Total noninterest expense	99	35
Income (loss) before income taxes	(508)	(925)
Applicable income tax expense (benefit)	(86)	(170)
Net Income	(422)	(755)
Total goodwill	0	0
Total assets	36,526	28,109
Impairment losses on bank premises		0
Bank premises and equipment held for sale	$ 16	$ 22