Loans and Allowance for Loan Losses - Summary of Recorded Investment in TDR's (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Pre- and Post - modification recorded investments
Commitment to lend additional funds to borrowers with loan modifications classified as TDRs	$ 49	$ 66
Total restructured loans	435	577
Commercial
Pre- and Post - modification recorded investments
Total restructured loans	218	337
Commercial | Commercial and industrial
Pre- and Post - modification recorded investments
Total restructured loans	109	202
Commercial | Commercial mortgage
Pre- and Post - modification recorded investments
Total restructured loans	46	7
Commercial | Construction
Pre- and Post - modification recorded investments
Total restructured loans	63	128
Consumer
Pre- and Post - modification recorded investments
Total restructured loans	217	240
Consumer | Residential mortgage
Pre- and Post - modification recorded investments
Total restructured loans	196	215
Consumer | Home equity and other consumer loans
Pre- and Post - modification recorded investments
Total restructured loans	$ 21	$ 25