Loans and Allowance for Loan Losses - Summary of Troubled Debt Restructurings (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Pre- and Post - modification recorded investments
Commitment to lend additional funds to borrowers with loan modifications classified as TDRs	$ 30	$ 59
Total excluding purchased credit-impaired loans	532	599
Commercial
Pre- and Post - modification recorded investments
Total excluding purchased credit-impaired loans	279	330
Commercial | Commercial and industrial
Pre- and Post - modification recorded investments
Total excluding purchased credit-impaired loans	214	321
Commercial | Commercial mortgage
Pre- and Post - modification recorded investments
Total excluding purchased credit-impaired loans	7	9
Commercial | Construction
Pre- and Post - modification recorded investments
Total excluding purchased credit-impaired loans	58	0
Consumer
Pre- and Post - modification recorded investments
Total excluding purchased credit-impaired loans	253	269
Consumer | Residential mortgage
Pre- and Post - modification recorded investments
Total excluding purchased credit-impaired loans	227	239
Consumer | Home equity and other consumer loans
Pre- and Post - modification recorded investments
Total excluding purchased credit-impaired loans	$ 26	$ 30