Loans and Allowance for Loan Losses - Summary of Recorded Investment in TDR's (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Pre- and Post - modification recorded investments
Commitment to lend additional funds to borrowers with loan modifications classified as TDRs	$ 66	$ 59
Total restructured loans	577	599
Commercial
Pre- and Post - modification recorded investments
Total restructured loans	337	330
Commercial | Commercial and industrial
Pre- and Post - modification recorded investments
Total restructured loans	202	321
Commercial | Commercial mortgage
Pre- and Post - modification recorded investments
Total restructured loans	7	9
Commercial | Construction
Pre- and Post - modification recorded investments
Total restructured loans	128	0
Consumer
Pre- and Post - modification recorded investments
Total restructured loans	240	269
Consumer | Residential mortgage
Pre- and Post - modification recorded investments
Total restructured loans	215	239
Consumer | Home equity and other consumer loans
Pre- and Post - modification recorded investments
Total restructured loans	$ 25	$ 30