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, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Derivative Instruments, Gain (Loss)
Gain (loss) on derivative instruments, net	$ 188,682	$ (177,244)	$ (344,733)	$ (201,460)	$ (293,485)	$ 87,672	$ 67,169	$ 133,367	$ (534,755)	$ (5,277)	$ 18,155
Interest Rate Swaps
Derivative Instruments, Gain (Loss)
Gain (loss) on derivative instruments, net									23,700	25,800	25,500
Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss)
Realized gain (loss) on derivative instruments, net									(597,077)	(2,830)	67,838
Contractual net interest income (expense)									35,840	(20,015)	(77,076)
Unrealized gain (loss), net									26,482	17,568	27,393
Gain (loss) on derivative instruments, net									(534,755)	(5,277)	18,155
Not Designated as Hedging Instrument | GSE CRT Embedded Derivatives
Derivative Instruments, Gain (Loss)
Realized gain (loss) on derivative instruments, net									0	0	0
Contractual net interest income (expense)									20,833	22,478	23,343
Unrealized gain (loss), net									(12,490)	(22,629)	28,305
Gain (loss) on derivative instruments, net									8,343	(151)	51,648
Not Designated as Hedging Instrument | Interest Rate Swaps
Derivative Instruments, Gain (Loss)
Realized gain (loss) on derivative instruments, net									(440,626)	81,417	72,894
Contractual net interest income (expense)									35,840	(20,015)	(77,076)
Unrealized gain (loss), net									18,826	24,358	28,316
Gain (loss) on derivative instruments, net									(385,960)	85,760	24,134
Not Designated as Hedging Instrument | Future Contracts
Derivative Instruments, Gain (Loss)
Realized gain (loss) on derivative instruments, net									(157,929)
Contractual net interest income (expense)									0
Unrealized gain (loss), net									7,836
Gain (loss) on derivative instruments, net									(150,093)
Not Designated as Hedging Instrument | Currency Forward Contracts
Derivative Instruments, Gain (Loss)
Realized gain (loss) on derivative instruments, net									1,478	2,088	(5,056)
Contractual net interest income (expense)									0	0	0
Unrealized gain (loss), net									(180)	1,046	(923)
Gain (loss) on derivative instruments, net									$ 1,298	3,134	$ (5,979)
Not Designated as Hedging Instrument | TBAs
Derivative Instruments, Gain (Loss)
Realized gain (loss) on derivative instruments, net										(17)
Contractual net interest income (expense)										0
Unrealized gain (loss), net										0
Gain (loss) on derivative instruments, net										(17)
Not Designated as Hedging Instrument | Future Contracts
Derivative Instruments, Gain (Loss)
Realized gain (loss) on derivative instruments, net										(86,318)
Contractual net interest income (expense)										0
Unrealized gain (loss), net										(7,836)
Gain (loss) on derivative instruments, net										$ (94,154)