Derivatives and Hedging Activities - Effect of Derivative Financial Instruments on Statement of Operations (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Derivative Instruments, Gain (Loss)
Contractual net interest income (expense)		$ 6,300	$ 20,800
Gain (loss) on derivative instruments, net	$ 122,611	(851,050)	(534,755)
Interest Rate Swaps
Derivative Instruments, Gain (Loss)
Gain (loss) on derivative instruments, net	(22,000)	(23,800)	(23,700)
Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss)
Realized gain (loss) on derivative instruments, net	156,157	(844,577)	(597,077)
Contractual net interest income (expense)	(15,803)	8,047	35,840
Unrealized gain (loss), net	(17,743)	(14,520)	26,482
Gain (loss) on derivative instruments, net	122,611	(851,050)	(534,755)
Not Designated as Hedging Instrument | GSE CRT Embedded Derivatives
Derivative Instruments, Gain (Loss)
Realized gain (loss) on derivative instruments, net		(31,354)	0
Contractual net interest income (expense)		6,323	20,833
Unrealized gain (loss), net		(10,281)	(12,490)
Gain (loss) on derivative instruments, net		(35,312)	8,343
Not Designated as Hedging Instrument | Interest Rate Swaps
Derivative Instruments, Gain (Loss)
Realized gain (loss) on derivative instruments, net	185,232	(857,753)	(440,626)
Contractual net interest income (expense)	(15,803)	8,047	35,840
Unrealized gain (loss), net	(5,869)	(24,068)	18,826
Gain (loss) on derivative instruments, net	163,560	(873,774)	(385,960)
Not Designated as Hedging Instrument | Futures 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 | Interest Rate Swaptions
Derivative Instruments, Gain (Loss)
Realized gain (loss) on derivative instruments, net	(553)
Contractual net interest income (expense)	0
Unrealized gain (loss), net	0
Gain (loss) on derivative instruments, net	(553)
Not Designated as Hedging Instrument | Currency Forward Contracts
Derivative Instruments, Gain (Loss)
Realized gain (loss) on derivative instruments, net	209	(1,301)	1,478
Contractual net interest income (expense)	0	0	0
Unrealized gain (loss), net	970	(345)	(180)
Gain (loss) on derivative instruments, net	1,179	(1,646)	$ 1,298
Not Designated as Hedging Instrument | TBAs
Derivative Instruments, Gain (Loss)
Realized gain (loss) on derivative instruments, net	(28,731)	14,477
Contractual net interest income (expense)	0	0
Unrealized gain (loss), net	(12,844)	9,893
Gain (loss) on derivative instruments, net	$ (41,575)	$ 24,370