Loans and Allowance for Loan Losses - Balance of Loans Held for Investment, by Class (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Aging of loans
Current	$ 76,976	$ 78,707
30 to 89 Days Past Due	216	143
90 Days or More Past Due	122	71
Total Past Due	338	214
Total	77,314	78,921
Loans still accruing	10	2
Commercial
Aging of loans
Current	43,835	48,276
30 to 89 Days Past Due	86	37
90 Days or More Past Due	65	13
Total Past Due	151	50
Total	43,986	48,326
Commercial | Commercial and industrial
Aging of loans
Current	27,049	32,104
30 to 89 Days Past Due	53	15
90 Days or More Past Due	54	6
Total Past Due	107	21
Total	27,156	32,125
Commercial | Commercial mortgage
Aging of loans
Current	14,503	13,880
30 to 89 Days Past Due	33	17
90 Days or More Past Due	11	7
Total Past Due	44	24
Total	14,547	13,904
Commercial | Construction
Aging of loans
Current	2,283	2,292
30 to 89 Days Past Due	0	5
90 Days or More Past Due	0	0
Total Past Due	0	5
Total	2,283	2,297
Consumer
Aging of loans
Current	33,141	30,431
30 to 89 Days Past Due	130	106
90 Days or More Past Due	57	58
Total Past Due	187	164
Total	33,328	30,595
Consumer | Residential mortgage
Aging of loans
Current	29,691	27,206
30 to 89 Days Past Due	104	92
90 Days or More Past Due	41	46
Total Past Due	145	138
Total	29,836	27,344
Consumer | Home equity and other consumer loans
Aging of loans
Current	3,450	3,225
30 to 89 Days Past Due	26	14
90 Days or More Past Due	16	12
Total Past Due	42	26
Total	3,492	3,251
Purchased Credit- Impaired
Aging of loans
Loans still accruing	$ 13	$ 16