Derivative and Other Hedging Instruments (TBA Securities by Coupon and Issuer) (Details) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Derivative [Line Items]
Forward Purchases of Agency MBS Specified Pools	$ 2,600
TBA and Forward Settling Agency Securities [Member]
Derivative [Line Items]
Derivative, Notional Amount	14,881	$ 7,295
Cost Basis	(15,540)	(7,430)
Net long TBA position, at fair value	(15,586)	(7,444)
TBA, net carrying value	46	14
Government National Mortgage Association Certificates and Obligations (GNMA) [Member] | TBA and Forward Settling Agency Securities [Member]
Derivative [Line Items]
Derivative, Notional Amount	473	573
Cost Basis	(489)	(582)
Net long TBA position, at fair value	(492)	(582)
TBA, net carrying value	3	0
Federal Home Loan Mortgage Corporation Certificates and Obligations (FHLMC) [Member] | TBA and Forward Settling Agency Securities [Member]
Derivative [Line Items]
Derivative, Notional Amount	1,648	689
Cost Basis	(1,723)	(703)
Net long TBA position, at fair value	(1,729)	(703)
TBA, net carrying value	6	0
Federal National Mortgage Association Certificates and Obligations (FNMA) [Member] | TBA and Forward Settling Agency Securities [Member]
Derivative [Line Items]
Derivative, Notional Amount	12,760	6,033
Cost Basis	(13,328)	(6,145)
Net long TBA position, at fair value	(13,365)	(6,159)
TBA, net carrying value	37	14
30 Year Maturity [Member] | TBA and Forward Settling Agency Securities [Member]
Derivative [Line Items]
Derivative, Notional Amount	12,654	7,014
Cost Basis	(13,243)	(7,135)
Net long TBA position, at fair value	(13,280)	(7,150)
TBA, net carrying value	37	15
30 Year Maturity [Member] | 3.0% Coupon [Member] | TBA and Forward Settling Agency Securities [Member]
Derivative [Line Items]
Derivative, Notional Amount	7,031	3,914
Cost Basis	(7,276)	(3,911)
Net long TBA position, at fair value	(7,303)	(3,916)
TBA, net carrying value	27	5
30 Year Maturity [Member] | 3.5% Coupon [Member] | TBA and Forward Settling Agency Securities [Member]
Derivative [Line Items]
Derivative, Notional Amount	3,156	1,497
Cost Basis	(3,322)	(1,536)
Net long TBA position, at fair value	(3,327)	(1,539)
TBA, net carrying value	5	3
30 Year Maturity [Member] | 4.0% Coupon [Member] | TBA and Forward Settling Agency Securities [Member]
Derivative [Line Items]
Derivative, Notional Amount	2,368	1,575
Cost Basis	(2,537)	(1,658)
Net long TBA position, at fair value	(2,542)	(1,665)
TBA, net carrying value	5	7
30 Year Maturity [Member] | 4.5% Coupon [Member] | TBA and Forward Settling Agency Securities [Member]
Derivative [Line Items]
Derivative, Notional Amount	99	28
Cost Basis	(108)	(30)
Net long TBA position, at fair value	(108)	(30)
TBA, net carrying value	0	0
15 Year Maturity [Member] | TBA and Forward Settling Agency Securities [Member]
Derivative [Line Items]
Derivative, Notional Amount	2,227	281
Cost Basis	(2,297)	(295)
Net long TBA position, at fair value	(2,306)	(294)
TBA, net carrying value	9	(1)
15 Year Maturity [Member] | 2.5% Coupon [Member] | TBA and Forward Settling Agency Securities [Member]
Derivative [Line Items]
Derivative, Notional Amount	1,839
Cost Basis	(1,891)
Net long TBA position, at fair value	(1,899)
TBA, net carrying value	8	1
15 Year Maturity [Member] | 3.0% Coupon [Member] | TBA and Forward Settling Agency Securities [Member]
Derivative [Line Items]
Derivative, Notional Amount	293	225
Cost Basis	(306)	(233)
Net long TBA position, at fair value	(307)	(232)
TBA, net carrying value	1	(1)
15 Year Maturity [Member] | 3.5% Coupon [Member] | TBA and Forward Settling Agency Securities [Member]
Derivative [Line Items]
Derivative, Notional Amount	95	136
Cost Basis	(100)	(143)
Net long TBA position, at fair value	(100)	(142)
TBA, net carrying value	$ 0	(1)
15 Year Maturity [Member] | Short [Member] | 2.5% Coupon [Member] | TBA and Forward Settling Agency Securities [Member]
Derivative [Line Items]
Derivative, Notional Amount		80
Cost Basis		(81)
Net long TBA position, at fair value		$ (80)