Loans and Allowance for Loan Losses - Summary of Troubled Debt Restructurings (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Pre- and Post - modification recorded investments
Commitment to lend additional funds to borrowers with loan modifications classified as TDRs	$ 38	$ 24
Financing Receivable, Troubled Debt Restructuring	463	491
Commercial Loans
Pre- and Post - modification recorded investments
Financing Receivable, Troubled Debt Restructuring	233	241
Consumer Loans
Pre- and Post - modification recorded investments
Financing Receivable, Troubled Debt Restructuring	230	250
Construction | Commercial Loans
Pre- and Post - modification recorded investments
Financing Receivable, Troubled Debt Restructuring	0	28
Residential mortgage | Consumer Loans
Pre- and Post - modification recorded investments
Financing Receivable, Troubled Debt Restructuring	229	248
Other consumer | Consumer Loans
Pre- and Post - modification recorded investments
Financing Receivable, Troubled Debt Restructuring	1	2
Commercial mortgage | Commercial Loans
Pre- and Post - modification recorded investments
Financing Receivable, Troubled Debt Restructuring	129	140
Commercial and industrial | Commercial Loans
Pre- and Post - modification recorded investments
Financing Receivable, Troubled Debt Restructuring	$ 104	$ 73