Business Segment Information - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Noninterest income:
Fixed income	$ 47	$ 28	$ 138	$ 97
Deposit transactions and cash management	45	46	134	133
Brokerage, management fees and commissions	27	21	76	66
Card and digital banking fees	19	20	58	61
Other service charges and fees	13	14	40	41
Trust services and investment management	12	12	36	35
Mortgage banking income	9	7	28	18
Gain on merger termination	0	0	0	225
Securities gains (losses), net	1	0	2	1
Other income	27	25	69	68
Total noninterest income	200	173	581	745
Underwriting, portfolio advisory, and other noninterest income
Noninterest income:
Revenue from contract with customer	11	13	31	32
Regional Banking
Noninterest income:
Fixed income	0	0	0	0
Deposit transactions and cash management	40	41	119	118
Brokerage, management fees and commissions	27	21	76	66
Card and digital banking fees	17	18	51	54
Other service charges and fees	6	6	18	18
Trust services and investment management	12	12	36	35
Mortgage banking income	0	0	0	0
Gain on merger termination				0
Securities gains (losses), net	0		0	0
Other income	12	8	28	25
Total noninterest income	114	106	328	316
Specialty Banking
Noninterest income:
Fixed income	47	28	138	98
Deposit transactions and cash management	3	3	9	9
Brokerage, management fees and commissions	0	0	0	0
Card and digital banking fees	0	1	2	2
Other service charges and fees	7	8	22	23
Trust services and investment management	0	0	0	0
Mortgage banking income	8	7	27	18
Gain on merger termination				0
Securities gains (losses), net	0		0	0
Other income	2	2	5	6
Total noninterest income	67	49	203	156
Corporate
Noninterest income:
Fixed income	0	0	0	(1)
Deposit transactions and cash management	2	2	6	6
Brokerage, management fees and commissions	0	0	0	0
Card and digital banking fees	2	1	5	5
Other service charges and fees	0	0	0	0
Trust services and investment management	0	0	0	0
Mortgage banking income	1	0	1	0
Gain on merger termination				225
Securities gains (losses), net	1		2	1
Other income	13	15	36	37
Total noninterest income	$ 19	$ 18	$ 50	$ 273