Business Segment Information - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Disaggregation of Revenue [Line Items]
Fixed income	$ 126	$ 96
Mortgage banking and title income	53	2
Deposit transactions and cash management	42	30
Brokerage, management fees and commissions	20	16
Trust services and investment management	12	7
Bankcard income	11	7
Other income	34	16
Total noninterest income	298	174
Regional Banking
Disaggregation of Revenue [Line Items]
Fixed income	1	0
Mortgage banking and title income	0	0
Deposit transactions and cash management	38	26
Brokerage, management fees and commissions	20	16
Trust services and investment management	12	7
Bankcard income	11	6
Other income	18	18
Total noninterest income	100	73
Specialty Banking
Disaggregation of Revenue [Line Items]
Fixed income	125	96
Mortgage banking and title income	52	2
Deposit transactions and cash management	3	3
Brokerage, management fees and commissions	0	0
Trust services and investment management	0	0
Bankcard income	0	1
Other income	5	2
Total noninterest income	185	104
Corporate
Disaggregation of Revenue [Line Items]
Fixed income	0	0
Mortgage banking and title income	1	0
Deposit transactions and cash management	1	1
Brokerage, management fees and commissions	0	0
Trust services and investment management	0	0
Bankcard income	0	0
Other income	11	(4)
Total noninterest income	13	(3)
Underwriting, portfolio advisory, and other noninterest income
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	$ 10	$ 9