Loans and Allowance for Loan Losses - Recorded Investment Amounts of Troubled Debt Restructurings (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Troubled debt restructured loans
Minimum defaulting period			60 days
Total troubled debt restructured loans at date of default	$ 3	$ 0	$ 31	$ 41
Commercial
Troubled debt restructured loans
Total troubled debt restructured loans at date of default	0	0	28	41
Consumer Portfolio Segment [Member]
Troubled debt restructured loans
Total troubled debt restructured loans at date of default	3		3
Commercial and industrial | Commercial Loans Portfolio Segment [Member]
Troubled debt restructured loans
Total troubled debt restructured loans at date of default			28	40
Commercial and industrial | Commercial | Commercial and industrial
Troubled debt restructured loans
Total troubled debt restructured loans at date of default	0	0
Commercial mortgage | Commercial Loans Portfolio Segment [Member]
Troubled debt restructured loans
Total troubled debt restructured loans at date of default				$ 1
Commercial mortgage | Commercial | Commercial and industrial
Troubled debt restructured loans
Total troubled debt restructured loans at date of default		$ 0
Residential mortgage and home equity | Consumer Loans Portfolio Segment [Member]
Troubled debt restructured loans
Total troubled debt restructured loans at date of default	$ 3		$ 3