Loans and Allowance for Loan Losses - Summary of Troubled Debt Restructurings (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Pre- and Post - modification recorded investments
Commitment to lend additional funds to borrowers with loan modifications classified as TDRs	$ 51	$ 49
Financing Receivable, Troubled Debt Restructuring	540	435
Commercial
Pre- and Post - modification recorded investments
Financing Receivable, Troubled Debt Restructuring	336	218
Consumer
Pre- and Post - modification recorded investments
Financing Receivable, Troubled Debt Restructuring	204	217
Construction | Commercial Loans
Pre- and Post - modification recorded investments
Financing Receivable, Troubled Debt Restructuring	62	63
Home equity and other consumer loans | Consumer Loans
Pre- and Post - modification recorded investments
Financing Receivable, Troubled Debt Restructuring	20	21
Residential mortgage | Consumer Loans
Pre- and Post - modification recorded investments
Financing Receivable, Troubled Debt Restructuring	184	196
Commercial mortgage | Commercial Loans
Pre- and Post - modification recorded investments
Financing Receivable, Troubled Debt Restructuring	104	46
Commercial and industrial | Commercial Loans
Pre- and Post - modification recorded investments
Financing Receivable, Troubled Debt Restructuring	$ 170	$ 109