Loans and Allowance for Loan Losses - Summary of Troubled Debt Restructurings (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Pre- and Post - modification recorded investments
Commitment to lend additional funds to borrowers with loan modifications classified as TDRs	$ 45	$ 49
Financing Receivable, Modifications, Recorded Investment	459	435
Commercial
Pre- and Post - modification recorded investments
Financing Receivable, Modifications, Recorded Investment	246	218
Consumer
Pre- and Post - modification recorded investments
Financing Receivable, Modifications, Recorded Investment	213	217
Construction | Commercial Loans
Pre- and Post - modification recorded investments
Financing Receivable, Modifications, Recorded Investment	62	63
Home equity and other consumer loans | Consumer Loans
Pre- and Post - modification recorded investments
Financing Receivable, Modifications, Recorded Investment	21	21
Residential mortgage | Consumer Loans
Pre- and Post - modification recorded investments
Financing Receivable, Modifications, Recorded Investment	192	196
Commercial mortgage | Commercial Loans
Pre- and Post - modification recorded investments
Financing Receivable, Modifications, Recorded Investment	47	46
Commercial and industrial | Commercial Loans
Pre- and Post - modification recorded investments
Financing Receivable, Modifications, Recorded Investment	$ 137	$ 109