Schedule of Derivative Instruments, Gain (Loss) in Statement of Financial Performance (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized in Income on Derivatives	$ 44,394	$ (19,131)	$ 239,800	$ (133,318)
Forward Contracts [Member] | Gain (Loss) on Derivative Instruments [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized in Income on Derivatives	(45,231)	(16,891)	(55,766)	(45,671)
Short US Treasury Securities [Member] | Gain (Loss) on Derivative Instruments [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized in Income on Derivatives	1,606	0	1,606	0
Options Held [Member] | Gain (Loss) on Derivative Instruments [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized in Income on Derivatives	13,489	(3,405)	43,328	(22,467)
Interest Rate Swaption [Member] | Gain (Loss) on Interest Rate Swap and Swaption Agreements [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized in Income on Derivatives	24,629	(12,349)	94,933	(62,080)
Interest Rate Cap [Member] | Gain (Loss) on Interest Rate Swap and Swaption Agreements [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized in Income on Derivatives	686	0	686	0
Total Return Swap [Member] | Gain (Loss) on Derivative Instruments [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized in Income on Derivatives	(302)	(134)	371	(821)
Inverse Interest-Only Securities [Member] | Gain (Loss) on Derivative Instruments [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized in Income on Derivatives	(1,025)	1,506	(5,274)	2,631
Short [Member] | Interest Rate Swap [Member] | Gain (Loss) on Interest Rate Swap and Swaption Agreements [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized in Income on Derivatives	(54,653)	(5,280)	(252,375)	22,813
Long [Member] | Interest Rate Swap [Member] | Gain (Loss) on Interest Rate Swap and Swaption Agreements [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized in Income on Derivatives	$ 105,195	$ 17,422	$ 412,291	$ (27,723)