Loans and Allowance for Loan Losses - Summary of Troubled Debt Restructurings (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Pre- and Post - modification recorded investments
Commitment to lend additional funds to borrowers with loan modifications classified as TDRs	$ 33	$ 61
Financing Receivable, Troubled Debt Restructuring	476	563
Commercial
Pre- and Post - modification recorded investments
Financing Receivable, Troubled Debt Restructuring	289	370
Consumer
Pre- and Post - modification recorded investments
Financing Receivable, Troubled Debt Restructuring	187	193
Construction | Commercial Loans
Pre- and Post - modification recorded investments
Financing Receivable, Troubled Debt Restructuring	62	62
Residential mortgage | Consumer Loans
Pre- and Post - modification recorded investments
Financing Receivable, Troubled Debt Restructuring	186	192
Other Consumer Loans [Member] | Consumer Loans
Pre- and Post - modification recorded investments
Financing Receivable, Troubled Debt Restructuring	1	1
Commercial mortgage | Commercial Loans
Pre- and Post - modification recorded investments
Financing Receivable, Troubled Debt Restructuring	168	168
Commercial and industrial | Commercial Loans
Pre- and Post - modification recorded investments
Financing Receivable, Troubled Debt Restructuring	$ 59	$ 140