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, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Troubled debt restructured loans
Minimum defaulting period			60 days
Total troubled debt restructured loans at date of default	$ 2	$ 17	$ 2	$ 21
Commercial
Troubled debt restructured loans
Total troubled debt restructured loans at date of default		17		20
Commercial | Commercial and industrial
Troubled debt restructured loans
Total troubled debt restructured loans at date of default		17
Commercial | Commercial Real Estate [Member]
Troubled debt restructured loans
Total troubled debt restructured loans at date of default		0
Consumer
Troubled debt restructured loans
Total troubled debt restructured loans at date of default	2	0	2	1
Consumer | Residential mortgage
Troubled debt restructured loans
Total troubled debt restructured loans at date of default	$ 2	$ 0
Commercial and industrial | Commercial Loans Portfolio Segment [Member]
Troubled debt restructured loans
Total troubled debt restructured loans at date of default				19
Commercial mortgage | Commercial Loans Portfolio Segment [Member]
Troubled debt restructured loans
Total troubled debt restructured loans at date of default				1
Residential mortgage | Consumer Loans
Troubled debt restructured loans
Total troubled debt restructured loans at date of default			$ 2	$ 1