Loans and Allowance for Loan Losses - Loans in Commercial Portfolio Segment Monitored for Credit Quality Based on Regulatory Risk Ratings (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Commercial
Aging of loans
Total loans	$ 40,830	$ 40,909
Commercial | Pass
Aging of loans
Total loans	39,332	39,345
Commercial | Special Mention
Aging of loans
Total loans	518	530
Commercial | Classified
Aging of loans
Total loans	980	1,034
Commercial | Commercial and industrial
Aging of loans
Total loans	24,478	24,814
Commercial | Commercial mortgage
Aging of loans
Total loans	14,790	14,320
Commercial | Construction
Aging of loans
Total loans	1,562	1,775
Commercial mortgage | Commercial Loans | Pass
Aging of loans
Total loans	14,532	14,081
Commercial mortgage | Commercial Loans | Special Mention
Aging of loans
Total loans	83	80
Commercial mortgage | Commercial Loans | Classified
Aging of loans
Total loans	175	159
Commercial and industrial | Commercial Loans | Pass
Aging of loans
Total loans	23,333	23,632
Commercial and industrial | Commercial Loans | Special Mention
Aging of loans
Total loans	418	435
Commercial and industrial | Commercial Loans | Classified
Aging of loans
Total loans	727	747
Construction | Commercial Loans | Pass
Aging of loans
Total loans	1,467	1,632
Construction | Commercial Loans | Special Mention
Aging of loans
Total loans	17	15
Construction | Commercial Loans | Classified
Aging of loans
Total loans	$ 78	$ 128