Loans and Allowance for Loan Losses - Summary of Troubled Debt Restructurings (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Pre- and Post - modification recorded investments
Commitment to lend additional funds to borrowers with loan modifications classified as TDRs	$ 36	$ 61
Financing Receivable, Troubled Debt Restructuring	570	563
Commercial
Pre- and Post - modification recorded investments
Financing Receivable, Troubled Debt Restructuring	369	370
Consumer
Pre- and Post - modification recorded investments
Financing Receivable, Troubled Debt Restructuring	201	193
Construction | Commercial Loans
Pre- and Post - modification recorded investments
Financing Receivable, Troubled Debt Restructuring	52	62
Residential mortgage | Consumer Loans
Pre- and Post - modification recorded investments
Financing Receivable, Troubled Debt Restructuring	200	192
Other consumer | 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	170	168
Commercial and industrial | Commercial Loans
Pre- and Post - modification recorded investments
Financing Receivable, Troubled Debt Restructuring	$ 147	$ 140