Securities - Available for Sale Securities (Unrealized Loss Position) (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Held-to-maturity securities:
Gross unrealized losses, less than 12 months	$ 1
Temporarily impaired securities, fair value, less than 12 months	726	$ 647
Temporarily impaired securities, gross unrealized losses, less than 12 months	1	2
Temporarily impaired securities, fair value, 12 months or longer	21,803	24,158
Temporarily impaired securities, gross unrealized losses, 12 months or longer	623	943
Temporarily impaired securities, fair value	22,529	24,805
Temporarily impaired securities, gross unrealized losses	624	945
US Treasury and Government [Member]
Securities available for sale:
Fair value, less than 12 months	0	0
Gross unrealized losses, less than 12 months	0	0
Fair value, 12 months or longer	148	147
Gross unrealized losses, 12 months or longer	2	3
Fair value	148	147
Gross unrealized losses	2	3
Collateralized Mortgage Obligations [Member]
Securities available for sale:
Fair value, less than 12 months	347	570
Gross unrealized losses, less than 12 months	1	2
Fair value, 12 months or longer	9,807	10,945
Gross unrealized losses, 12 months or longer	241	371
Fair value	10,154	11,515
Gross unrealized losses	242	373
Held-to-maturity securities:
Fair value, less than 12 months	77	0
Gross unrealized losses, less than 12 months		0
Fair value, 12 months or longer	6,092	6,416
Gross unrealized losses, 12 months or longer	175	254
Fair value	6,169	6,416
Gross unrealized losses	175	254
Commercial Mortgage Backed Securities [Member]
Securities available for sale:
Fair value, less than 12 months	0	0
Gross unrealized losses, less than 12 months	0	0
Fair value, 12 months or longer	1,742	1,729
Gross unrealized losses, 12 months or longer	108	132
Fair value	1,742	1,729
Gross unrealized losses		132
Held-to-maturity securities:
Fair value, less than 12 months	203	73
Gross unrealized losses, less than 12 months		1
Fair value, 12 months or longer	2,593	3,359
Gross unrealized losses, 12 months or longer	69	133
Fair value	2,796	3,432
Gross unrealized losses	69	133
Other Securities [Member]
Held-to-maturity securities:
Fair value, less than 12 months	3
Fair value, 12 months or longer	0
Gross unrealized losses, 12 months or longer	0
Fair value	3
Gross unrealized losses	0
Other Mortgage-Backed Securities [Member] | Residential Mortgage Backed Securities [Member]
Securities available for sale:
Fair value, less than 12 months	1	4
Gross unrealized losses, less than 12 months	1	1
Fair value, 12 months or longer	1,047	1,087
Gross unrealized losses, 12 months or longer	22	36
Fair value	1,048	1,091
Gross unrealized losses	22	36
Held-to-maturity securities:
Fair value, less than 12 months	95	0
Gross unrealized losses, less than 12 months		0
Fair value, 12 months or longer	374	475
Gross unrealized losses, 12 months or longer	6	14
Fair value	469	475
Gross unrealized losses	6	$ 14
Other Mortgage-Backed Securities [Member] | Commercial Mortgage Backed Securities [Member]
Securities available for sale:
Gross unrealized losses	$ 108