Loans and Allowance for Loan Losses - Balance of Loans Held for Investment, by Class (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Aging of loans
Current	$ 77,049	$ 76,013
30 to 89 Days Past Due	143	174
Total loans 90 days or more past due and still accruing	71	92
Total Past Due	214	266
Total loans held for investment, before purchased credit-impaired loans	77,263	76,279
Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing	2	3
Commercial portfolio
Aging of loans
Current	46,618	44,127
30 to 89 Days Past Due	37	42
Total loans 90 days or more past due and still accruing	13	16
Total Past Due	50	58
Total loans held for investment, before purchased credit-impaired loans	46,668	44,185
Commercial portfolio | Commercial and industrial
Aging of loans
Current	30,446	28,392
30 to 89 Days Past Due	15	19
Total loans 90 days or more past due and still accruing	6	12
Total Past Due	21	31
Total loans held for investment, before purchased credit-impaired loans	30,467	28,423
Commercial portfolio | Commercial mortgage
Aging of loans
Current	13,880	13,991
30 to 89 Days Past Due	17	21
Total loans 90 days or more past due and still accruing	7	4
Total Past Due	24	25
Total loans held for investment, before purchased credit-impaired loans	13,904	14,016
Commercial portfolio | Construction
Aging of loans
Current	2,292	1,744
30 to 89 Days Past Due	5	2
Total loans 90 days or more past due and still accruing	0	0
Total Past Due	5	2
Total loans held for investment, before purchased credit-impaired loans	2,297	1,746
Consumer portfolio
Aging of loans
Current	30,431	31,886
30 to 89 Days Past Due	106	132
Total loans 90 days or more past due and still accruing	58	76
Total Past Due	164	208
Total loans held for investment, before purchased credit-impaired loans	30,595	32,094
Consumer portfolio | Residential mortgage
Aging of loans
Current	27,206	28,802
30 to 89 Days Past Due	92	112
Total loans 90 days or more past due and still accruing	46	63
Total Past Due	138	175
Total loans held for investment, before purchased credit-impaired loans	27,344	28,977
Consumer portfolio | Home equity and other consumer loans
Aging of loans
Current	3,225	3,084
30 to 89 Days Past Due	14	20
Total loans 90 days or more past due and still accruing	12	13
Total Past Due	26	33
Total loans held for investment, before purchased credit-impaired loans	3,251	3,117
Purchased credit-impaired loans
Aging of loans
Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing	$ 16	$ 47