Loans and Allowance for Loan Losses - Aging of Balance of Loans Held for Investment By Class (Details) - USD ($) $ in Millions		Jun. 30, 2018	Dec. 31, 2017
Aging of loans
Current		$ 81,920	$ 79,687
Total past due		316	327
Loans held for investment	[1]	82,236	80,014
Loans 90 days or more past due and still accruing		10	12
30 to 89 Days Past Due
Aging of loans
Total past due		192	207
90 Days or More Past Due
Aging of loans
Total past due		124	120
Commercial
Aging of loans
Current		40,717	40,807
Total past due		113	102
Loans held for investment		40,830	40,909
Commercial | Commercial and industrial
Aging of loans
Total past due		94	80
Loans held for investment		24,478	24,814
Commercial | Commercial mortgage
Aging of loans
Total past due		19	22
Loans held for investment		14,790	14,320
Commercial | Construction
Aging of loans
Total past due		0	0
Loans held for investment		1,562	1,775
Commercial | 30 to 89 Days Past Due
Aging of loans
Total past due		33	33
Commercial | 90 Days or More Past Due
Aging of loans
Total past due		80	69
Consumer
Aging of loans
Current		41,203	38,880
Total past due		203	225
Loans held for investment		41,406	39,105
Consumer | Residential mortgage
Aging of loans
Total past due		172	190
Loans held for investment		37,552	35,643
Consumer | Home equity and other consumer loans
Aging of loans
Total past due		31	35
Loans held for investment		3,854	3,462
Consumer | 30 to 89 Days Past Due
Aging of loans
Total past due		159	174
Consumer | 90 Days or More Past Due
Aging of loans
Total past due		44	51
Home equity and other consumer loans | Consumer Loans
Aging of loans
Current		3,823	3,427
30 to 89 Days Past Due		20	23
90 Days or More Past Due		11	12
Residential mortgage | Consumer Loans
Aging of loans
Current		37,380	35,453
30 to 89 Days Past Due		139	151
90 Days or More Past Due		33	39
Commercial mortgage | Commercial Loans
Aging of loans
Current		14,771	14,298
30 to 89 Days Past Due		17	16
90 Days or More Past Due		2	6
Commercial and industrial | Commercial Loans
Aging of loans
Current		24,384	24,734
30 to 89 Days Past Due		16	17
90 Days or More Past Due		78	63
Construction | Commercial Loans
Aging of loans
Current		1,562	1,775
30 to 89 Days Past Due		0	0
90 Days or More Past Due		$ 0	$ 0

[1] Includes $326 million and $301 million at June 30, 2018 and December 31, 2017, respectively, for net unamortized (discounts) and premiums and deferred (fees) and costs.