Investment Securities (Summary of Investment in Agency Security) (Details) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Amortized cost	$ 45,982	$ 52,147
Non-Agency Securities Unrealized Gain	2	0
Non-Agency Securities Unrealized Loss	0	(1)
Mortgage Backed Securities Amortized Cost	46,304	52,464
Mortgage Backed Securities Unrealized Gain	1,065	407
Mortgage Backed Securities Unrealized Loss	(13)	(398)
Gross unrealized gain	1,063	407
Gross unrealized loss	(13)	(397)
Fair value	46,993	52,120
Credit Risk Transfer Securities Unrealized Gain	0
Credit Risk Transfer Securities Unrealized Loss	0
Total agency MBS, amortized cost	46,168	52,350
Total agency MBS, at fair value	47,218	52,360
Mortgage-backed Securities Available-for-sale, Fair Value Disclosure	47,356	52,473
Mortgage-backed Securities, Issued by Private Enterprises [Member]
Amortized cost	100	114
Fair value	102
Interest-only and principal-only strips, fair value		113
Credit Risk Transfer Securities [Member]
Interest-only and principal-only strips, amortized cost	36
Interest-only and principal-only strips, fair value	36
Fixed Income Securities [Member]
Gross unrealized gain	980	339
Gross unrealized loss	(10)	(393)
Fixed Income Securities [Member] | Agency Securities [Member]
Amortized cost	44,641	50,576
Fair value	45,611	50,522
Adjustable-Rate [Member]
Gross unrealized gain	14	11
Gross unrealized loss	0	0
Adjustable-Rate [Member] | Agency Securities [Member]
Amortized cost	401	484
Fair value	415	495
Collateralized Mortgage Obligations [Member]
Gross unrealized gain	25	18
Gross unrealized loss	0	1
Collateralized Mortgage Obligations [Member] | Agency Securities [Member]
Amortized cost	840	973
Fair value	865	990
Interest Only And Principal Only Strip [Member]
Gross unrealized gain	44	39
Gross unrealized loss	(3)	3
Interest Only And Principal Only Strip [Member] | Agency Securities [Member]
Interest-only and principal-only strips, amortized cost	286	317
Interest-only and principal-only strips, fair value	$ 327	$ 353