Derivative and Other Hedging Instruments (TBA Securities by Coupon and Issuer) (Details) - TBA and Forward Settling Agency Securities [Member] - USD ($) $ in Millions	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018
Derivative [Line Items]
Derivative, Notional Amount	$ 19,760	$ 20,279	$ 7,322	$ 11,043	$ 6,822	$ 7,152
Cost Basis	20,413		7,404
Net long TBA position, at fair value	(20,543)		(7,429)
TBA, net carrying value	130		25
30 Year Maturity [Member]
Derivative [Line Items]
Derivative, Notional Amount	12,389		5,142
Cost Basis	12,812		5,182
Net long TBA position, at fair value	(12,907)		(5,203)
TBA, net carrying value	95		21
30 Year Maturity [Member] | 2.5% Coupon [Member]
Derivative [Line Items]
Derivative, Notional Amount	10,758		0
Cost Basis	11,091		0
Net long TBA position, at fair value	(11,187)		0
TBA, net carrying value	96		0
30 Year Maturity [Member] | 3.0% Coupon [Member]
Derivative [Line Items]
Derivative, Notional Amount	908		5,008
Cost Basis	951		5,052
Net long TBA position, at fair value	(955)		(5,073)
TBA, net carrying value	4		21
30 Year Maturity [Member] | 3.5% Coupon [Member]
Derivative [Line Items]
Derivative, Notional Amount	99		1,226
Cost Basis	105		1,259
Net long TBA position, at fair value	(104)		(1,261)
TBA, net carrying value	(1)		2
30 Year Maturity [Member] | 4.0% Coupon [Member]
Derivative [Line Items]
Derivative, Notional Amount	624
Cost Basis	665		1,565
Net long TBA position, at fair value	(661)		(1,568)
TBA, net carrying value	(4)		(3)
30 Year Maturity [Member] | 4.5% Coupon [Member]
Derivative [Line Items]
Derivative, Notional Amount	0		415
Cost Basis	0		436
Net long TBA position, at fair value	0		(437)
TBA, net carrying value	0		1
30 Year Maturity [Member] | Short [Member] | 4.0% Coupon [Member]
Derivative [Line Items]
Derivative, Notional Amount			1,507
15 Year Maturity [Member]
Derivative [Line Items]
Derivative, Notional Amount	7,371		2,180
Cost Basis	7,601		2,222
Net long TBA position, at fair value	(7,636)		(2,226)
TBA, net carrying value	35		4
15 Year Maturity [Member] | 2.5% Coupon [Member]
Derivative [Line Items]
Derivative, Notional Amount	7,296		805
Cost Basis	7,522		811
Net long TBA position, at fair value	(7,557)		(812)
TBA, net carrying value	35		1
15 Year Maturity [Member] | 3.0% Coupon [Member]
Derivative [Line Items]
Derivative, Notional Amount	75		1,059
Cost Basis			1,083
Net long TBA position, at fair value	(79)		(1,086)
TBA, net carrying value	0		3
15 Year Maturity [Member] | 3.5% Coupon [Member]
Derivative [Line Items]
Derivative, Notional Amount	0		241
Cost Basis	0		250
Net long TBA position, at fair value	0		(250)
TBA, net carrying value	0		0
15 Year Maturity [Member] | 4.0% Coupon [Member]
Derivative [Line Items]
Derivative, Notional Amount	0		75
Cost Basis	0		78
Net long TBA position, at fair value	0		(78)
TBA, net carrying value	0		$ 0
15 Year Maturity [Member] | Short [Member] | 3.0% Coupon [Member]
Derivative [Line Items]
Cost Basis	$ 79