Securities - Available for Sale Securities (Unrealized Loss Position) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Held-to-maturity Securities, Gross unrealized losses	$ 272	$ 227
Fair value less than 12 months, total temporarily impaired securities	10,991	24,038
Gross unrealized losses less than 12 months, total temporarily impaired securities	125	497
Fair value 12 months or longer, total temporarily impaired securities	16,041	1,931
Gross unrealized losses 12 months or longer, total temporarily impaired securities	570	62
Fair value, total temporarily impaired securities	27,032	25,969
Gross unrealized losses, total temporarily impaired securities	695	559
US Treasury and Government [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	41	182
Gross unrealized losses, less than 12 months	0	4
Fair value, 12 months or longer	116	0
Gross unrealized losses, 12 months or longer	2	0
Fair Value	157	182
Gross unrealized losses, Total	2	4
Held-to-maturity Securities, Fair Value less than 12 months	0
Held-to-maturity Securities, Gross unrealized losses, less than 12 months	0
Held-to-maturity Securities, Fair Value 12 months or longer	0
Held-to-maturity Securities, Gross unrealized losses, 12 months or longer	0
Held-to-maturity Securities, Fair Value	0
Held-to-maturity Securities, Gross unrealized losses	0
States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	0
Gross unrealized losses, less than 12 months	0
Fair value, 12 months or longer	0
Gross unrealized losses, 12 months or longer	0
Fair Value	0
Gross unrealized losses, Total	0
Held-to-maturity Securities, Fair Value less than 12 months	0
Held-to-maturity Securities, Gross unrealized losses, less than 12 months	0
Held-to-maturity Securities, Fair Value 12 months or longer	0
Held-to-maturity Securities, Gross unrealized losses, 12 months or longer	0
Held-to-maturity Securities, Fair Value	0
Held-to-maturity Securities, Gross unrealized losses	0
Collateralized Mortgage Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	6,153	12,345
Gross unrealized losses, less than 12 months	74	231
Fair value, 12 months or longer	7,270	1,410
Gross unrealized losses, 12 months or longer	261	44
Fair Value	13,423	13,755
Gross unrealized losses, Total	335	275
Held-to-maturity Securities, Fair Value less than 12 months	2,201	7,028
Held-to-maturity Securities, Gross unrealized losses, less than 12 months	27	156
Held-to-maturity Securities, Fair Value 12 months or longer	5,599	518
Held-to-maturity Securities, Gross unrealized losses, 12 months or longer	197	17
Held-to-maturity Securities, Fair Value	7,800	7,546
Held-to-maturity Securities, Gross unrealized losses	224	173
Residential Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Held-to-maturity Securities, Gross unrealized losses	4	5
Residential Mortgage Backed Securities [Member] | Other Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	666	1,452
Gross unrealized losses, less than 12 months	7	17
Fair value, 12 months or longer	702	0
Gross unrealized losses, 12 months or longer	13	0
Fair Value	1,368	1,452
Gross unrealized losses, Total	20	17
Held-to-maturity Securities, Fair Value less than 12 months	252	547
Held-to-maturity Securities, Gross unrealized losses, less than 12 months	1	5
Held-to-maturity Securities, Fair Value 12 months or longer	206	0
Held-to-maturity Securities, Gross unrealized losses, 12 months or longer	3	0
Held-to-maturity Securities, Fair Value	458	547
Held-to-maturity Securities, Gross unrealized losses	4	5
Commercial Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	205	1,482
Gross unrealized losses, less than 12 months	4	35
Fair value, 12 months or longer	1,649	0
Gross unrealized losses, 12 months or longer	62	0
Fair Value	1,854	1,482
Gross unrealized losses, Total		35
Held-to-maturity Securities, Fair Value less than 12 months	1,470	996
Held-to-maturity Securities, Gross unrealized losses, less than 12 months	12	49
Held-to-maturity Securities, Fair Value 12 months or longer	495	0
Held-to-maturity Securities, Gross unrealized losses, 12 months or longer	32	0
Held-to-maturity Securities, Fair Value	1,965	996
Held-to-maturity Securities, Gross unrealized losses	44	49
Commercial Mortgage Backed Securities [Member] | Other Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross unrealized losses, Total	66
Held-to-maturity Securities, Gross unrealized losses	44	49
Other Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	0	2
Gross unrealized losses, less than 12 months	0	0
Fair value, 12 months or longer	0	3
Gross unrealized losses, 12 months or longer	0	1
Fair Value	0	5
Gross unrealized losses, Total	0	1
Held-to-maturity Securities, Fair Value less than 12 months	3	4
Held-to-maturity Securities, Gross unrealized losses, less than 12 months	0	0
Held-to-maturity Securities, Fair Value 12 months or longer	4	0
Held-to-maturity Securities, Gross unrealized losses, 12 months or longer	0	0
Held-to-maturity Securities, Fair Value	7	4
Held-to-maturity Securities, Gross unrealized losses	0	0
Gross unrealized losses, available for sale securities		1
Gross unrealized losses, held-to-maturity securities	$ 1	$ 1