Loans and Allowance for Loan Losses - Loans in Commercial Portfolio Segment Monitored for Credit Quality Based on Regulatory Risk Ratings (Details) - Commercial - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016	[1]
Aging of loans
Total loans	$ 41,256	$ 43,769
Pass
Aging of loans
Total loans	39,401	41,342
Special Mention
Aging of loans
Total loans	717	1,142
Classified
Aging of loans
Total loans	1,138	1,285
Commercial and industrial
Aging of loans
Total loans	25,239	26,985
Commercial and industrial | Pass
Aging of loans
Total loans	23,872	25,028
Commercial and industrial | Special Mention
Aging of loans
Total loans	616	860
Commercial and industrial | Classified
Aging of loans
Total loans	751	1,097
Commercial mortgage
Aging of loans
Total loans	14,161	14,501
Commercial mortgage | Pass
Aging of loans
Total loans	13,836	14,152
Commercial mortgage | Special Mention
Aging of loans
Total loans	87	161
Commercial mortgage | Classified
Aging of loans
Total loans	238	188
Construction
Aging of loans
Total loans	1,856	2,283
Construction | Pass
Aging of loans
Total loans	1,693	2,162
Construction | Special Mention
Aging of loans
Total loans	14	121
Construction | Classified
Aging of loans
Total loans	$ 149	$ 0

[1] The amounts presented reflect unpaid principal balances less charge-offs.