Schedule of Derivative Instruments, Gain (Loss) in Statement of Financial Performance (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Interest Rate Derivative Instruments Not Designated as Hedging Instruments	$ 44,952	$ (116,019)	$ (81,491)	$ (221,547)
Derivative Instruments, Gain (Loss) Recognized in Income, Net	(5,484)	(24,202)	(2,517)	(18,401)
Gain (Loss) on Interest Rate Swap and Swaption Agreements			35,258	(182,420)
Gain (Loss) on Mortgage Loans Held-for-Sale	(6,832)	11,801	2,260	8,620
Trading Activity, Gains and Losses, Net	34,338	(136,058)	(88,168)	(236,202)
Agency To-be-Announced Securities, Non-Risk Management [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	0	0	0	(4,701)
Inverse Interest-Only Securities [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	4,909	16,052	24,170	34,375
Forward Purchase Commitments [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Mortgage Loans Held-for-Sale	(5,130)	4,163	(4,160)	3,746
Interest Rate Risk Associated with the Investment Portfolio [Member] | Forward Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	5,262	(29,877)	(22,696)	(47,780)
Interest Rate Risk Associated with the Investment Portfolio [Member] | Short US Treasury Securities [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	125	(8)	125	(8)
Interest Rate Risk Associated with the Investment Portfolio [Member] | Options Held [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	5,671	(4,614)	8,206	(6,319)
Interest Rate Risk Associated with the Investment Portfolio [Member] | Treasury Lock [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	(837)	0	(837)	0
Interest Rate Risk Associated with the Investment Portfolio [Member] | Swap [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	74	(6,103)	6,164	5,428
Interest Rate Risk Associated with the Investment Portfolio [Member] | Interest Rate Swaption [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Interest Rate Derivative Instruments Not Designated as Hedging Instruments	19,752	(57,250)	1,246	(169,808)
Interest Rate Risk Associated with the Investment Portfolio [Member] | Total Return Swap [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	(20,658)	353	(17,526)	(1,372)
Interest Rate Risk Associated with the Investment Portfolio [Member] | Short [Member] | Interest Rate Swap [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Interest Rate Derivative Instruments Not Designated as Hedging Instruments	(42,470)	65,963	(144)	106,942
Interest Rate Risk Associated with the Investment Portfolio [Member] | Long [Member] | Interest Rate Swap [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Interest Rate Derivative Instruments Not Designated as Hedging Instruments	20,713	(46,341)	(31,846)	(59,761)
Interest Rate Risk Associated with Borrowings [Member] | Long [Member] | Interest Rate Swap [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Interest Rate Derivative Instruments Not Designated as Hedging Instruments	46,957	(78,391)	(50,747)	(98,920)
Credit Risk Associated with Non-Agency RMBS and Mortgage Loans Held-for-Sale [Member] | Credit Default Swap, Selling Protection [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	$ (30)	$ (5)	$ (123)	$ 1,976