Loans and Allowance for Loan Losses - Recorded Investment Amounts of TDRs at Date of Default (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Pre- and Post - modification recorded investments
Total troubled debt restructured loans at date of default	$ 33	$ 27
Minimum defaulting period	60 days
Commercial portfolio
Pre- and Post - modification recorded investments
Total troubled debt restructured loans at date of default	$ 29	18
Commercial portfolio | Commercial and industrial
Pre- and Post - modification recorded investments
Total troubled debt restructured loans at date of default	25	10
Commercial portfolio | Commercial mortgage
Pre- and Post - modification recorded investments
Total troubled debt restructured loans at date of default	4	8
Consumer portfolio
Pre- and Post - modification recorded investments
Total troubled debt restructured loans at date of default	4	9
Consumer portfolio | Residential mortgage
Pre- and Post - modification recorded investments
Total troubled debt restructured loans at date of default	4	8
Consumer portfolio | Home equity and other consumer loans
Pre- and Post - modification recorded investments
Total troubled debt restructured loans at date of default	$ 0	$ 1