Derivative and Other Hedging Instruments (Narrative) (Details) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) month	Jun. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)	Mar. 31, 2017 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Forward Contracts [Member] | Interest Rate Swap [Member]
Average Maturity (Years)			9 years 9 months 18 days		10 years 8 months 12 days
Interest Rate Swaption [Member]
Average Maturity (Years)			9 years 6 months
Cash Flow Hedges Derivative Instruments Not Designated As Hedging Instruments Average Months To Expiration | month			12
Notional Amount	$ 4,950		$ 4,950
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	(13)	$ (4)	(24)	$ (11)
Notional Amount Of Derivatives Not Designated As Hedging Instruments Additions During The Period	2,750	0	3,750	0
Notional Amount Of Derivatives Not Designated As Hedging Instruments Settlement Expiration During The Period	$ 0	(700)	$ 0	(1,100)
Derivative, Average Fixed Interest Rate	2.79%		2.79%
Interest Rate Derivatives, at Fair Value, Net	$ 86		$ 86
Options At Cost			141
TBA and Forward Settling Agency Securities [Member]
Notional Amount	16,867	6,756	16,867	6,756	$ 10,916	$ 14,170	$ 5,813	$ 7,295
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	163	108	202	324
Notional Amount Of Derivatives Not Designated As Hedging Instruments Additions During The Period	56,306	20,066	92,402	38,025
Notional Amount Of Derivatives Not Designated As Hedging Instruments Settlement Expiration During The Period		(19,123)		(38,564)
US Treasury Securities [Member]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	(16)	(44)	(28)	(121)
Unrealized gain on derivative instruments, net	0	12	0	31
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	(170)	(368)	(209)	(1,319)
Discontinuation of Election to Account for Interest Rate Swaps as Designated Cash Flow Hedges [Member]
Net Unrealized Gain (Loss) on Swaps	0	8	0	8	0	0	20	39
Not Designated as Hedging Instrument [Member] | Interest Rate Swaption [Member]
Notional Amount	4,950		4,950
Short [Member] | US Treasury Securities [Member]
Notional Amount	7,358	2,930	7,358	2,930	8,061	$ 9,169	3,135	1,714
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	(110)	(73)	(188)	(156)
Notional Amount Of Derivatives Not Designated As Hedging Instruments Additions During The Period	3,932	(653)	(6,490)	2,633
Notional Amount Of Derivatives Not Designated As Hedging Instruments Settlement Expiration During The Period	(5,743)	858	7,193	(1,417)
Long [Member] | US Treasury Securities [Member]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net		1	1	6
Trading Securities Added During the Period		225	303	405
Notional Amount Of Trading Securities Settlement Expiration During The Period		(163)	(492)	(368)
Trading Securities		$ 62		$ 62	$ 189		$ 0	$ 25
Long [Member]
Trading Securities	$ 0		$ 0