Loans and Allowance for Loan Losses - Summary of Recorded Investment in TDR's (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Pre- and Post - modification recorded investments
Commitment to lend additional funds to borrowers with loan modifications classified as TDRs	$ 61	$ 49
Total restructured loans	563	435
Commercial | Commercial and industrial
Pre- and Post - modification recorded investments
Total restructured loans	140	109
Commercial | Commercial mortgage
Pre- and Post - modification recorded investments
Total restructured loans	168	46
Commercial | Construction
Pre- and Post - modification recorded investments
Total restructured loans	62	63
Consumer | Residential mortgage
Pre- and Post - modification recorded investments
Total restructured loans	192	216
Consumer | Other Consumer Loans [Member]
Pre- and Post - modification recorded investments
Total restructured loans	1	1
Consumer Portfolio Segment [Member]
Pre- and Post - modification recorded investments
Total restructured loans	193	217
Commercial Portfolio Segment [Member]
Pre- and Post - modification recorded investments
Total restructured loans	$ 370	$ 218