Loans and Allowance for Loan Losses - Recorded Investment Amounts of Troubled Debt Restructurings (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Troubled debt restructured loans
Minimum defaulting period			60 days
Total troubled debt restructured loans at date of default	$ 40	$ 2	$ 41	$ 2
Commercial
Troubled debt restructured loans
Total troubled debt restructured loans at date of default	40		41
Consumer
Troubled debt restructured loans
Total troubled debt restructured loans at date of default	0	2	0	2
Home Equity and Other Consumer Loans [Member] | Consumer Loans
Troubled debt restructured loans
Total troubled debt restructured loans at date of default	0		0
Commercial and industrial | Commercial Loans Portfolio Segment [Member]
Troubled debt restructured loans
Total troubled debt restructured loans at date of default	40		40
Commercial mortgage | Commercial Loans Portfolio Segment [Member]
Troubled debt restructured loans
Total troubled debt restructured loans at date of default	0		1
Residential mortgage | Consumer Loans
Troubled debt restructured loans
Total troubled debt restructured loans at date of default	$ 0	$ 2	$ 0	$ 2