Derivatives and Hedging Activities - Effect of Derivative Financial Instruments on Statement of Operations (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Derivative Instruments, Gain (Loss)
Gain (loss) on derivative instruments, net	$ 133,367	$ 5,462
Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss)
Realized gain (loss) on derivative instruments, net	113,578	14,918
Contractual net interest expense	(12,112)	(22,894)
Unrealized gain (loss), net	31,901	13,438
Gain (loss) on derivative instruments, net	133,367	5,462
Not Designated as Hedging Instrument | GSE CRT embedded derivatives
Derivative Instruments, Gain (Loss)
Realized gain (loss) on derivative instruments, net	0	0
Contractual net interest expense	5,633	5,807
Unrealized gain (loss), net	(2,468)	14,148
Gain (loss) on derivative instruments, net	3,165	19,955
Not Designated as Hedging Instrument | Interest Rate Swaps
Derivative Instruments, Gain (Loss)
Realized gain (loss) on derivative instruments, net	122,273	15,994
Contractual net interest expense	(12,112)	(22,894)
Unrealized gain (loss), net	32,374	12,950
Gain (loss) on derivative instruments, net	142,535	6,050
Not Designated as Hedging Instrument | Futures Contracts
Derivative Instruments, Gain (Loss)
Realized gain (loss) on derivative instruments, net	(5,277)
Contractual net interest expense	0
Unrealized gain (loss), net	(1,612)
Gain (loss) on derivative instruments, net	(6,889)
Not Designated as Hedging Instrument | Currency Forward Contracts
Derivative Instruments, Gain (Loss)
Realized gain (loss) on derivative instruments, net	(3,418)	(1,076)
Contractual net interest expense	0	0
Unrealized gain (loss), net	1,139	488
Gain (loss) on derivative instruments, net	$ (2,279)	$ (588)