Mortgage-Backed and Credit Risk Transfer Securities - Summary of Investment Portfolio (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Principal/ Notional Balance	$ 8,203,415,000	$ 8,557,917,000
Unamortized Premium (Discount)	(343,684,000)	(413,441,000)
Amortized Cost	7,859,731,000	8,144,476,000
Allowance for Credit Losses	0	(1,768,000)	$ 0
Unrealized Gain/ (Loss), net	(55,472,000)	29,474,000
Fair Value	$ 7,804,259,000	$ 8,172,182,000
Period- end Weighted Average Yield	2.14%	1.97%
Unamortized premium (discount) non-accretable portion	$ 2,100,000	$ 2,100,000
Percentage of Non-Agency RMBS interest-only, principal balance	97.70%	98.80%
Percentage of Non-Agency RMBS interest-only, amortized cost	44.80%	49.30%
Percentage of Non-Agency RMBS interest only, fair value	19.90%	41.50%
30 year fixed-rate
Debt Securities, Available-for-sale [Line Items]
Principal/ Notional Balance	$ 7,514,229,000	$ 7,635,107,000
Unamortized Premium (Discount)	246,183,000	391,644,000
Amortized Cost	7,760,412,000	8,026,751,000
Allowance for Credit Losses		0
Unrealized Gain/ (Loss), net	(58,889,000)	24,115,000
Fair Value	$ 7,701,523,000	$ 8,050,866,000
Period- end Weighted Average Yield	2.07%	1.86%
Total Agency RMBS pass-through
Debt Securities, Available-for-sale [Line Items]
Principal/ Notional Balance	$ 7,514,229,000	$ 7,635,107,000
Unamortized Premium (Discount)	246,183,000	391,644,000
Amortized Cost	7,760,412,000	8,026,751,000
Allowance for Credit Losses		0
Unrealized Gain/ (Loss), net	(58,889,000)	24,115,000
Fair Value	$ 7,701,523,000	$ 8,050,866,000
Period- end Weighted Average Yield	2.07%	1.86%
Agency-CMO
Debt Securities, Available-for-sale [Line Items]
Principal/ Notional Balance	$ 235,216,000	$ 19,634,000
Unamortized Premium (Discount)	(203,180,000)	(19,634,000)
Amortized Cost	32,036,000	0
Allowance for Credit Losses		0
Unrealized Gain/ (Loss), net	(1,279,000)	0
Fair Value	$ 30,757,000	$ 0
Period- end Weighted Average Yield	6.47%	0.00%
Non-Agency CMBS
Debt Securities, Available-for-sale [Line Items]
Principal/ Notional Balance	$ 61,427,000	$ 112,549,000
Unamortized Premium (Discount)	(3,096,000)	(5,791,000)
Amortized Cost	58,331,000	106,758,000
Allowance for Credit Losses		(1,768,000)
Unrealized Gain/ (Loss), net	4,578,000	4,593,000
Fair Value	$ 62,909,000	$ 109,583,000
Period- end Weighted Average Yield	8.63%	9.40%
Non-Agency RMBS
Debt Securities, Available-for-sale [Line Items]
Principal/ Notional Balance	$ 392,543,000	$ 790,627,000
Unamortized Premium (Discount)	(383,591,000)	(779,660,000)
Amortized Cost	8,952,000	10,967,000
Allowance for Credit Losses		0
Unrealized Gain/ (Loss), net	118,000	766,000
Fair Value	$ 9,070,000	$ 11,733,000
Period- end Weighted Average Yield	5.26%	7.83%
Percentage of non-agency securities classified as variable rate	35.60%	31.80%
Percentage of non-agency securities classified as fixed rate	63.50%	67.30%
Percentage of non-agency securities classified as floating rate	0.90%	0.90%