Loans and Allowance for Loan Losses - Loans in Commercial Portfolio Segment Monitored for Credit Quality Based on Regulatory Risk Ratings (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Commercial
Aging of loans
Total loans	$ 43,615	$ 43,135
Commercial | Pass
Aging of loans
Total loans	41,875	41,871
Commercial | Special Mention
Aging of loans
Total loans	801	468
Commercial | Classified
Aging of loans
Total loans	939	796
Commercial | Commercial and industrial
Aging of loans
Total loans	26,625	26,168
Commercial | Commercial mortgage
Aging of loans
Total loans	15,299	15,354
Commercial | Construction
Aging of loans
Total loans	1,691	1,613
Commercial mortgage | Commercial Loans | Pass
Aging of loans
Total loans	15,018	15,138
Commercial mortgage | Commercial Loans | Special Mention
Aging of loans
Total loans	121	105
Commercial mortgage | Commercial Loans | Classified
Aging of loans
Total loans	160	111
Commercial and industrial | Commercial Loans | Pass
Aging of loans
Total loans	25,339	25,191
Commercial and industrial | Commercial Loans | Special Mention
Aging of loans
Total loans	569	355
Commercial and industrial | Commercial Loans | Classified
Aging of loans
Total loans	717	622
Construction | Commercial Loans | Pass
Aging of loans
Total loans	1,518	1,542
Construction | Commercial Loans | Special Mention
Aging of loans
Total loans	111	8
Construction | Commercial Loans | Classified
Aging of loans
Total loans	$ 62	$ 63