Loans and Allowance for Loan Losses - Aging of Balance of Loans Held for Investment By Class (Details) - USD ($) $ in Millions		Sep. 30, 2017	Dec. 31, 2016
Aging of loans
Current		$ 78,495	$ 77,188
Total past due		334	363
Loans held for investment	[1]	78,829	77,551
Loans 90 days or more past due and still accruing		13	23
30 to 89 Days Past Due
Aging of loans
Total past due		193	228
90 Days or More Past Due
Aging of loans
Total past due		141	135
Commercial
Aging of loans
Current		41,112	43,939
Total past due		144	167
Loans held for investment		41,256	44,106
Commercial | Commercial and industrial
Aging of loans
Current		25,115	27,085
Total past due		124	113
Loans held for investment		25,239	27,198
Commercial | Commercial mortgage
Aging of loans
Current		14,141	14,571
Total past due		20	54
Loans held for investment		14,161	14,625
Commercial | Construction
Aging of loans
Current		1,856	2,283
Total past due		0	0
Loans held for investment		1,856	2,283
Commercial | 30 to 89 Days Past Due
Aging of loans
Total past due		53	91
Commercial | 30 to 89 Days Past Due | Commercial and industrial
Aging of loans
Total past due		41	54
Commercial | 30 to 89 Days Past Due | Commercial mortgage
Aging of loans
Total past due		12	37
Commercial | 30 to 89 Days Past Due | Construction
Aging of loans
Total past due		0	0
Commercial | 90 Days or More Past Due
Aging of loans
Total past due		91	76
Commercial | 90 Days or More Past Due | Commercial and industrial
Aging of loans
Total past due		83	59
Commercial | 90 Days or More Past Due | Commercial mortgage
Aging of loans
Total past due		8	17
Commercial | 90 Days or More Past Due | Construction
Aging of loans
Total past due		0	0
Consumer
Aging of loans
Current		37,383	33,249
Total past due		190	196
Loans held for investment		37,573	33,445
Consumer | Residential mortgage
Aging of loans
Current		34,047	29,770
Total past due		158	152
Loans held for investment		34,205	29,922
Consumer | Home equity and other consumer loans
Aging of loans
Current		3,336	3,479
Total past due		32	44
Loans held for investment		3,368	3,523
Consumer | 30 to 89 Days Past Due
Aging of loans
Total past due		140	137
Consumer | 30 to 89 Days Past Due | Residential mortgage
Aging of loans
Total past due		120	110
Consumer | 30 to 89 Days Past Due | Home equity and other consumer loans
Aging of loans
Total past due		20	27
Consumer | 90 Days or More Past Due
Aging of loans
Total past due		50	59
Consumer | 90 Days or More Past Due | Residential mortgage
Aging of loans
Total past due		38	42
Consumer | 90 Days or More Past Due | Home equity and other consumer loans
Aging of loans
Total past due		$ 12	$ 17

[1] Includes $277 million and $180 million at September 30, 2017 and December 31, 2016, respectively, for net unamortized (discounts) and premiums and deferred (fees) and costs.