Loans and Allowance for Loan Losses - Summary of Troubled Debt Restructurings (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Pre- and Post - modification recorded investments
Commitment to lend additional funds to borrowers with loan modifications classified as TDRs	$ 40	$ 66
Financing Receivable, Modifications, Recorded Investment	576	577
Commercial
Pre- and Post - modification recorded investments
Financing Receivable, Modifications, Recorded Investment	351	337
Consumer
Pre- and Post - modification recorded investments
Financing Receivable, Modifications, Recorded Investment	225	240
Construction | Commercial Loans
Pre- and Post - modification recorded investments
Financing Receivable, Modifications, Recorded Investment	78	128
Home equity and other consumer loans | Consumer Loans
Pre- and Post - modification recorded investments
Financing Receivable, Modifications, Recorded Investment	23	25
Residential mortgage | Consumer Loans
Pre- and Post - modification recorded investments
Financing Receivable, Modifications, Recorded Investment	202	215
Commercial mortgage | Commercial Loans
Pre- and Post - modification recorded investments
Financing Receivable, Modifications, Recorded Investment	54	7
Commercial and industrial | Commercial Loans
Pre- and Post - modification recorded investments
Financing Receivable, Modifications, Recorded Investment	$ 219	$ 202