Investment Securities (Gross Unrealized Loss and Fair Value of Securities Available for Sale) (Details) - Debt Securities - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
AFS unrealized loss position less than 12 months - unrealized loss	$ (110)	$ (410)
AFS unrealized loss position less than 12 months - fair value	17,837	25,760
AFS unrealized loss position 12 months or more - unrealized loss	(336)	(126)
AFS unrealized loss position 12 months or more - fair value	14,697	6,287
Total AFS unrealized loss	(446)	(536)
Total AFS fair value	32,534	32,047
HTM unrealized loss position less than 12 months - unrealized loss	(25)	(187)
HTM unrealized loss position less than 12 months - fair value	3,445	8,425
HTM unrealized loss position 12 months or more - unrealized loss	(147)	(10)
HTM unrealized loss position 12 months or more - fair value	6,490	613
Total HTM unrealized loss	(172)	(197)
Total HTM fair value	9,935	9,038
US Treasury and Government Agencies Securities
Schedule of Available-for-sale Securities [Line Items]
AFS unrealized loss position less than 12 months - unrealized loss	(42)	(57)
AFS unrealized loss position less than 12 months - fair value	6,099	3,108
AFS unrealized loss position 12 months or more - unrealized loss	(42)	(20)
AFS unrealized loss position 12 months or more - fair value	1,465	2,028
Total AFS unrealized loss	(84)	(77)
Total AFS fair value	7,564	5,136
HTM unrealized loss position less than 12 months - unrealized loss	(3)	(22)
HTM unrealized loss position less than 12 months - fair value	195	238
HTM unrealized loss position 12 months or more - unrealized loss	(10)
HTM unrealized loss position 12 months or more - fair value	255
Total HTM unrealized loss	(13)	(22)
Total HTM fair value	450	238
Residential mortgage-backed Securities | Mortgage-backed Securities Agency
Schedule of Available-for-sale Securities [Line Items]
AFS unrealized loss position less than 12 months - unrealized loss	(47)	(267)
AFS unrealized loss position less than 12 months - fair value	8,151	16,942
AFS unrealized loss position 12 months or more - unrealized loss	(202)	(20)
AFS unrealized loss position 12 months or more - fair value	9,954	922
Total AFS unrealized loss	(249)	(287)
Total AFS fair value	18,105	17,864
HTM unrealized loss position less than 12 months - unrealized loss	(10)	(153)
HTM unrealized loss position less than 12 months - fair value	3,167	8,041
HTM unrealized loss position 12 months or more - unrealized loss	(129)	(8)
HTM unrealized loss position 12 months or more - fair value	6,168	161
Total HTM unrealized loss	(139)	(161)
Total HTM fair value	9,335	8,202
Residential mortgage-backed Securities | Mortgage-backed Securities Non-agency
Schedule of Available-for-sale Securities [Line Items]
AFS unrealized loss position less than 12 months - unrealized loss		(1)
AFS unrealized loss position less than 12 months - fair value		109
AFS unrealized loss position 12 months or more - unrealized loss	(21)	(51)
AFS unrealized loss position 12 months or more - fair value	383	1,119
Total AFS unrealized loss	(21)	(52)
Total AFS fair value	383	1,228
Commercial Mortgage Backed Securities | Mortgage-backed Securities Agency
Schedule of Available-for-sale Securities [Line Items]
AFS unrealized loss position less than 12 months - unrealized loss	(11)	(33)
AFS unrealized loss position less than 12 months - fair value	524	1,577
AFS unrealized loss position 12 months or more - unrealized loss	(47)	(1)
AFS unrealized loss position 12 months or more - fair value	1,302	86
Total AFS unrealized loss	(58)	(34)
Total AFS fair value	1,826	1,663
Commercial Mortgage Backed Securities | Mortgage-backed Securities Non-agency
Schedule of Available-for-sale Securities [Line Items]
AFS unrealized loss position less than 12 months - unrealized loss	(3)	(14)
AFS unrealized loss position less than 12 months - fair value	400	880
AFS unrealized loss position 12 months or more - unrealized loss	(6)	(13)
AFS unrealized loss position 12 months or more - fair value	333	987
Total AFS unrealized loss	(9)	(27)
Total AFS fair value	733	1,867
Asset backed
Schedule of Available-for-sale Securities [Line Items]
AFS unrealized loss position less than 12 months - unrealized loss	(4)	(5)
AFS unrealized loss position less than 12 months - fair value	1,697	1,317
AFS unrealized loss position 12 months or more - unrealized loss	(4)	(17)
AFS unrealized loss position 12 months or more - fair value	462	902
Total AFS unrealized loss	(8)	(22)
Total AFS fair value	2,159	2,219
HTM unrealized loss position less than 12 months - unrealized loss
HTM unrealized loss position less than 12 months - fair value
HTM unrealized loss position 12 months or more - unrealized loss		(2)
HTM unrealized loss position 12 months or more - fair value		451
Total HTM unrealized loss		(2)
Total HTM fair value		451
Other debt
Schedule of Available-for-sale Securities [Line Items]
AFS unrealized loss position less than 12 months - unrealized loss	(3)	(33)
AFS unrealized loss position less than 12 months - fair value	966	1,827
AFS unrealized loss position 12 months or more - unrealized loss	(14)	(4)
AFS unrealized loss position 12 months or more - fair value	798	243
Total AFS unrealized loss	(17)	(37)
Total AFS fair value	1,764	2,070
HTM unrealized loss position less than 12 months - unrealized loss	(12)	(12)
HTM unrealized loss position less than 12 months - fair value	83	146
HTM unrealized loss position 12 months or more - unrealized loss	(8)
HTM unrealized loss position 12 months or more - fair value	67	1
Total HTM unrealized loss	(20)	(12)
Total HTM fair value	$ 150	$ 147