Loans and Allowance for Loan Losses - Summary of Recorded Investment in TDR's (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Pre- and Post - modification recorded investments
Commitment to lend additional funds to borrowers with loan modifications classified as TDRs	$ 98	$ 33
Total excluding purchased credit-impaired loans	822	466
Commercial portfolio
Pre- and Post - modification recorded investments
Total excluding purchased credit-impaired loans	515	128
Commercial portfolio | Commercial and industrial
Pre- and Post - modification recorded investments
Total excluding purchased credit-impaired loans	499	100
Commercial portfolio | Commercial mortgage
Pre- and Post - modification recorded investments
Total excluding purchased credit-impaired loans	16	28
Consumer portfolio
Pre- and Post - modification recorded investments
Total excluding purchased credit-impaired loans	307	338
Consumer portfolio | Residential mortgage
Pre- and Post - modification recorded investments
Total excluding purchased credit-impaired loans	276	308
Consumer portfolio | Home equity and other consumer loans
Pre- and Post - modification recorded investments
Total excluding purchased credit-impaired loans	$ 31	$ 30