Mortgage-Backed and Credit Risk Transfer Securities - Summary of Investment Portfolio (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Principal/ Notional Balance	$ 23,354,075	$ 20,114,683
Unamortized Premium (Discount)	(2,321,799)	(2,752,848)
Amortized Cost	21,032,276	17,361,835
Unrealized Gain/ (Loss), net	739,510	34,807
Fair Value	$ 21,771,786	$ 17,396,642
Period- end Weighted Average Yield	3.85%	4.00%
Percentage of Agency-CMO interest only securities, principal balance	56.30%	73.60%
Percentage of Agency-CMO interest only securities, amortized cost	6.40%	13.50%
Percentage of Agency-CMO interest only securities, fair value	6.40%	12.40%
Percentage of Non-Agency CMBS interest only, principal balance	13.10%	15.00%
Percentage of Non-Agency CMBS interest only, amortized cost	0.30%	0.40%
Percentage of Non-Agency CMBS interest only, fair value	0.30%	0.50%
Percentage of Non-Agency RMBS interest-only, principal balance		55.40%
Percentage of Non-Agency RMBS interest-only, amortized cost		2.30%
Percentage of Non-Agency RMBS interest only, fair value		2.40%
Unamortized premium (discount) non-accretable portion		$ 145,600
15 year fixed-rate
Debt Securities, Available-for-sale [Line Items]
Principal/ Notional Balance	$ 280,426	417,233
Unamortized Premium (Discount)	1,666	5,077
Amortized Cost	282,092	422,310
Unrealized Gain/ (Loss), net	10,322	1,944
Fair Value	$ 292,414	$ 424,254
Period- end Weighted Average Yield	3.34%	3.27%
30 year fixed-rate
Debt Securities, Available-for-sale [Line Items]
Principal/ Notional Balance	$ 9,911,339	$ 9,599,301
Unamortized Premium (Discount)	308,427	298,693
Amortized Cost	10,219,766	9,897,994
Unrealized Gain/ (Loss), net	304,454	(125,225)
Fair Value	$ 10,524,220	$ 9,772,769
Period- end Weighted Average Yield	3.62%	3.55%
Hybrid ARM
Debt Securities, Available-for-sale [Line Items]
Principal/ Notional Balance	$ 55,024	$ 653,586
Unamortized Premium (Discount)	602	13,775
Amortized Cost	55,626	667,361
Unrealized Gain/ (Loss), net	1,267	(7,413)
Fair Value	$ 56,893	$ 659,948
Period- end Weighted Average Yield	3.46%	2.79%
Agency RMBS
Debt Securities, Available-for-sale [Line Items]
Principal/ Notional Balance	$ 10,246,789	$ 10,670,120
Unamortized Premium (Discount)	310,695	317,545
Amortized Cost	10,557,484	10,987,665
Unrealized Gain/ (Loss), net	316,043	(130,694)
Fair Value	$ 10,873,527	$ 10,856,971
Period- end Weighted Average Yield	3.61%	3.49%
Agency-CMO
Debt Securities, Available-for-sale [Line Items]
Principal/ Notional Balance	$ 883,122	$ 907,862
Unamortized Premium (Discount)	(467,840)	(631,180)
Amortized Cost	415,282	276,682
Unrealized Gain/ (Loss), net	12,230	(8,991)
Fair Value	$ 427,512	$ 267,691
Period- end Weighted Average Yield	3.54%	3.61%
Agency CMBS
Debt Securities, Available-for-sale [Line Items]
Principal/ Notional Balance	$ 4,561,276	$ 973,122
Unamortized Premium (Discount)	75,299	15,058
Amortized Cost	4,636,575	988,180
Unrealized Gain/ (Loss), net	131,355	14,330
Fair Value	$ 4,767,930	$ 1,002,510
Period- end Weighted Average Yield	3.01%	3.54%
Unsettled TBA securities
Debt Securities, Available-for-sale [Line Items]
Principal/ Notional Balance	$ 99,300
Non-Agency CMBS
Debt Securities, Available-for-sale [Line Items]
Principal/ Notional Balance	4,464,525	$ 4,024,715
Unamortized Premium (Discount)	(772,295)	(727,307)
Amortized Cost	3,692,230	3,297,408
Unrealized Gain/ (Loss), net	131,244	(10,949)
Fair Value	$ 3,823,474	$ 3,286,459
Period- end Weighted Average Yield	5.16%	5.05%
Non-Agency RMBS
Debt Securities, Available-for-sale [Line Items]
Principal/ Notional Balance	$ 2,340,119	$ 2,800,335
Unamortized Premium (Discount)	(1,487,603)	(1,748,223)
Amortized Cost	852,516	1,052,112
Unrealized Gain/ (Loss), net	103,155	111,570
Fair Value	$ 955,671	$ 1,163,682
Period- end Weighted Average Yield	6.98%	7.24%
Percentage of non-agency securities classified as variable rate	37.00%	43.50%
Percentage of non-agency securities classified as fixed rate	57.70%	50.70%
Percentage of non-agency securities classified as floating rate	5.30%	5.80%
Unamortized premium (discount) non-accretable portion	$ 120,200
Non-Agency IO
Debt Securities, Available-for-sale [Line Items]
Percentage of Non-Agency RMBS interest-only, principal balance	56.20%
Percentage of Non-Agency RMBS interest-only, amortized cost	1.90%
Percentage of Non-Agency RMBS interest only, fair value	1.30%
GSE CRT
Debt Securities, Available-for-sale [Line Items]
Principal/ Notional Balance	$ 858,244	$ 738,529
Unamortized Premium (Discount)	19,945	21,259
Amortized Cost	878,189	759,788
Unrealized Gain/ (Loss), net	45,483	59,541
Fair Value	$ 923,672	$ 819,329
Period- end Weighted Average Yield	2.78%	3.10%