Loans and Allowance for Loan Losses - Purchased Credit-Impaired Loans Segment (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Aging of loans
Total loans		$ 48,131
Commercial Loan
Aging of loans
Total loans	$ 43,769
Consumer Loan
Aging of loans
Loans not covered	11	19
Commercial
Aging of loans
Total loans	43,696	48,018
Commercial | Commercial and industrial
Aging of loans
Total loans	26,975	31,963
Commercial | Commercial mortgage
Aging of loans
Total loans	14,438	13,758
Commercial | Construction
Aging of loans
Total loans	2,283	2,297
Purchased Credit- Impaired
Aging of loans
Total loans		113
Purchased Credit- Impaired | Commercial Loan
Aging of loans
Total loans	73
Pass
Aging of loans
Total loans		45,586
Pass | Commercial Loan
Aging of loans
Total loans	41,342
Pass | Commercial
Aging of loans
Total loans	41,311	45,546
Pass | Commercial | Commercial and industrial
Aging of loans
Total loans	25,028	29,836
Pass | Commercial | Commercial mortgage
Aging of loans
Total loans	14,121	13,470
Pass | Commercial | Construction
Aging of loans
Total loans	2,162	2,240
Pass | Purchased Credit- Impaired
Aging of loans
Total loans		40
Pass | Purchased Credit- Impaired | Commercial Loan
Aging of loans
Total loans	31
Special Mention
Aging of loans
Total loans		1,052
Special Mention | Commercial Loan
Aging of loans
Total loans	1,142
Special Mention | Commercial
Aging of loans
Total loans	1,138	1,040
Special Mention | Commercial | Commercial and industrial
Aging of loans
Total loans	860	844
Special Mention | Commercial | Commercial mortgage
Aging of loans
Total loans	157	139
Special Mention | Commercial | Construction
Aging of loans
Total loans	121	57
Special Mention | Purchased Credit- Impaired
Aging of loans
Total loans		12
Special Mention | Purchased Credit- Impaired | Commercial Loan
Aging of loans
Total loans	4
Classified
Aging of loans
Total loans		1,493
Classified | Commercial Loan
Aging of loans
Total loans	1,285
Classified | Commercial
Aging of loans
Total loans	1,247	1,432
Classified | Commercial | Commercial and industrial
Aging of loans
Total loans	1,087	1,283
Classified | Commercial | Commercial mortgage
Aging of loans
Total loans	160	149
Classified | Commercial | Construction
Aging of loans
Total loans	0	0
Classified | Purchased Credit- Impaired
Aging of loans
Total loans		61
Classified | Purchased Credit- Impaired | Commercial Loan
Aging of loans
Total loans	38
Oil and gas | Criticized | Commercial
Aging of loans
Total loans	$ 1,100	$ 1,200