Derivative and Other Hedging Instruments (TBA Securities by Coupon and Issuer) (Details) - TBA and Forward Settling Agency Securities [Member] - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
Derivative [Line Items]
Derivative, Notional Amount	$ 20,279	$ 7,322	$ 6,822	$ 7,152
Cost Basis	(20,648)	(7,404)
Net long TBA position, at fair value	(21,222)	(7,429)
TBA, net carrying value	574	25
30 Year Maturity [Member]
Derivative [Line Items]
Derivative, Notional Amount	20,179	5,142
Cost Basis	(20,560)	(5,182)
Net long TBA position, at fair value	(21,118)	(5,203)
TBA, net carrying value	558	21
30 Year Maturity [Member] | 2.5% Coupon [Member]
Derivative [Line Items]
Derivative, Notional Amount	5,786	0
Cost Basis	(5,884)	0
Net long TBA position, at fair value	(5,974)	0
TBA, net carrying value	90	0
30 Year Maturity [Member] | 3.0% Coupon [Member]
Derivative [Line Items]
Derivative, Notional Amount	9,072	5,008
Cost Basis	(9,191)	(5,052)
Net long TBA position, at fair value	(9,502)	(5,073)
TBA, net carrying value	311	21
30 Year Maturity [Member] | 3.5% Coupon [Member]
Derivative [Line Items]
Derivative, Notional Amount	3,366	1,226
Cost Basis	(3,451)	(1,259)
Net long TBA position, at fair value	(3,556)	(1,261)
TBA, net carrying value	105	2
30 Year Maturity [Member] | 4.0% Coupon [Member]
Derivative [Line Items]
Derivative, Notional Amount	1,955
Cost Basis	(2,034)	(1,565)
Net long TBA position, at fair value	(2,086)	(1,568)
TBA, net carrying value	52	(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	100	2,180
Cost Basis	(88)	(2,222)
Net long TBA position, at fair value	(104)	(2,226)
TBA, net carrying value	16	4
15 Year Maturity [Member] | 2.5% Coupon [Member]
Derivative [Line Items]
Derivative, Notional Amount	100	805
Cost Basis	(92)	(811)
Net long TBA position, at fair value	(104)	(812)
TBA, net carrying value	12	1
15 Year Maturity [Member] | 3.0% Coupon [Member]
Derivative [Line Items]
Derivative, Notional Amount	0	1,059
Cost Basis		(1,083)
Net long TBA position, at fair value	0	(1,086)
TBA, net carrying value	4	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	$ (4)