Derivatives and Hedging Activities - Effect of Derivative Financial Instruments on Statement of Operations (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Derivative Instruments, Gain (Loss)
Realized gain (loss) on derivative instruments, net									$ 67,838	$ (57,943)	$ (44,272)
Unrealized gain (loss), net									27,393	99,932	9,597
Gain (loss) on derivative instruments, net	$ 64,251	$ 1,955	$ (53,513)	$ 5,462	$ 230,713	$ 35,378	$ (90,363)	$ (238,543)	18,155	(62,815)	(219,048)
Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss)
Realized gain (loss) on derivative instruments, net									67,838	(57,943)	(44,272)
Contractual net interest expense									(77,076)	(104,804)	(184,373)
Unrealized gain (loss), net									27,393	99,932	9,597
Gain (loss) on derivative instruments, net									18,155	(62,815)	(219,048)
Not Designated as Hedging Instrument | Credit Derivatives
Derivative Instruments, Gain (Loss)
Realized gain (loss) on derivative instruments, net											(1,140)
Contractual net interest expense											25,545
Unrealized gain (loss), net											(4,623)
Gain (loss) on derivative instruments, net											19,782
Not Designated as Hedging Instrument | GSE CRT Embedded Derivatives
Derivative Instruments, Gain (Loss)
Realized gain (loss) on derivative instruments, net									0	(6,017)	(2,184)
Contractual net interest expense									23,343	24,343	24,822
Unrealized gain (loss), net									28,305	42,817	(4,227)
Gain (loss) on derivative instruments, net									51,648	61,143	18,411
Not Designated as Hedging Instrument | Credit Default Swap Contract
Derivative Instruments, Gain (Loss)
Realized gain (loss) on derivative instruments, net											1,044
Contractual net interest expense											723
Unrealized gain (loss), net											(396)
Gain (loss) on derivative instruments, net											1,371
Not Designated as Hedging Instrument | Interest Rate Swaps
Derivative Instruments, Gain (Loss)
Realized gain (loss) on derivative instruments, net									72,894	(69,090)	(31,881)
Contractual net interest expense									(77,076)	(104,804)	(184,373)
Unrealized gain (loss), net									28,316	100,503	(554)
Gain (loss) on derivative instruments, net									24,134	(73,391)	(216,808)
Not Designated as Hedging Instrument | Interest Rate Swaptions
Derivative Instruments, Gain (Loss)
Realized gain (loss) on derivative instruments, net										(1,485)	(10,328)
Contractual net interest expense										0	0
Unrealized gain (loss), net										1,485	8,521
Gain (loss) on derivative instruments, net										0	(1,807)
Not Designated as Hedging Instrument | TBAs
Derivative Instruments, Gain (Loss)
Realized gain (loss) on derivative instruments, net											(2,292)
Contractual net interest expense											0
Unrealized gain (loss), net											558
Gain (loss) on derivative instruments, net											(1,734)
Not Designated as Hedging Instrument | Futures Contracts
Derivative Instruments, Gain (Loss)
Realized gain (loss) on derivative instruments, net											(943)
Contractual net interest expense											0
Unrealized gain (loss), net											(90)
Gain (loss) on derivative instruments, net											(1,033)
Not Designated as Hedging Instrument | Currency Forward Contracts
Derivative Instruments, Gain (Loss)
Realized gain (loss) on derivative instruments, net									(5,056)	12,632	1,172
Contractual net interest expense									0	0	0
Unrealized gain (loss), net									(923)	(2,056)	1,162
Gain (loss) on derivative instruments, net									$ (5,979)	$ 10,576	$ 2,334