Loans and Allowance for Loan Losses - Recorded Investment Amounts of TDRs at Date of Default (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Pre- and Post - modification recorded investments
Total troubled debt restructured loans at date of default	$ 3	$ 23
Minimum defaulting period	60 days
Commercial
Pre- and Post - modification recorded investments
Total troubled debt restructured loans at date of default	$ 0	20
Commercial | Commercial and industrial
Pre- and Post - modification recorded investments
Total troubled debt restructured loans at date of default	0	19
Commercial | Commercial mortgage
Pre- and Post - modification recorded investments
Total troubled debt restructured loans at date of default	0	1
Consumer
Pre- and Post - modification recorded investments
Total troubled debt restructured loans at date of default	3	3
Consumer | Residential mortgage
Pre- and Post - modification recorded investments
Total troubled debt restructured loans at date of default	3	3
Consumer | Home equity and other consumer loans
Pre- and Post - modification recorded investments
Total troubled debt restructured loans at date of default	$ 0	$ 0