Loans and Allowance for Loan Losses - Purchased Credit-Impaired Loans Segment (Details) - Commercial - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Aging of loans
Total loans	$ 40,909	$ 43,769
Commercial and industrial
Aging of loans
Total loans	24,814	26,985
Commercial mortgage
Aging of loans
Total loans	14,320	14,501
Construction
Aging of loans
Total loans	1,775	2,283
Pass
Aging of loans
Total loans	39,345	41,342
Pass | Commercial and industrial
Aging of loans
Total loans	23,632	25,028
Pass | Commercial mortgage
Aging of loans
Total loans	14,081	14,152
Pass | Construction
Aging of loans
Total loans	1,632	2,162
Special Mention
Aging of loans
Total loans	530	1,142
Special Mention | Commercial and industrial
Aging of loans
Total loans	435	860
Special Mention | Commercial mortgage
Aging of loans
Total loans	80	161
Special Mention | Construction
Aging of loans
Total loans	15	121
Classified
Aging of loans
Total loans	1,034	1,285
Classified | Commercial and industrial
Aging of loans
Total loans	747	1,097
Classified | Commercial mortgage
Aging of loans
Total loans	159	188
Classified | Construction
Aging of loans
Total loans	$ 128	$ 0