Mortgage-Backed and Credit Risk Transfer Securities - Summary of Investment Portfolio (Detail) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Principal/ Notional Balance	$ 20,464,863	$ 20,855,353
Unamortized Premium (Discount)	(2,746,713)	(2,840,992)
Amortized Cost	17,718,150	18,014,361
Unrealized Gain/ (Loss), net	(95,916)	176,393
Fair value	$ 17,622,234	$ 18,190,754
Period- end Weighted Average Yield	3.48%	3.42%
Quarterly Weighted Average Yield	3.32%	3.27%
Percentage of agency collateralized mortgage obligations interest only securities, principal balance	81.40%	81.80%
Percentage of agency collateralized mortgage obligations interest only securities, amortized cost	19.40%	20.90%
Percentage of agency collateralized mortgage obligations interest only securities, fair value	17.60%	18.70%
Percentage of CMBS interest only, principal balance	15.60%	15.80%
Percentage of CMBS interest only, amortized cost	0.50%	0.50%
Percentage of CMBS interest only, fair value	0.60%	0.60%
Unamortized premium (discount) non-accretable portion	$ 191,100	$ 195,300
Percentage of Non-Agency RMBS interest-only, principal balance	51.90%	51.50%
Percentage of Non-Agency RMBS interest-only, amortized cost	2.00%	2.00%
Percentage of Non-Agency RMBS interest only, fair value	1.80%	1.80%
Agency RMBS
Schedule of Available-for-sale Securities [Line Items]
Principal/ Notional Balance	$ 11,995,105	$ 12,206,152
Unamortized Premium (Discount)	419,275	442,772
Amortized Cost	12,414,380	12,648,924
Unrealized Gain/ (Loss), net	(291,586)	(73,017)
Fair value	$ 12,122,794	$ 12,575,907
Period- end Weighted Average Yield	2.85%	2.79%
Quarterly Weighted Average Yield	2.65%	2.58%
Agency RMBS | 15 year fixed-rate
Schedule of Available-for-sale Securities [Line Items]
Principal/ Notional Balance	$ 2,603,748	$ 2,917,307
Unamortized Premium (Discount)	102,652	119,120
Amortized Cost	2,706,400	3,036,427
Unrealized Gain/ (Loss), net	(96,848)	(61,645)
Fair value	$ 2,609,552	$ 2,974,782
Period- end Weighted Average Yield	2.22%	2.17%
Quarterly Weighted Average Yield	2.04%	1.98%
Agency RMBS | 30 year fixed-rate
Schedule of Available-for-sale Securities [Line Items]
Principal/ Notional Balance	$ 7,546,645	$ 7,354,211
Unamortized Premium (Discount)	290,890	295,977
Amortized Cost	7,837,535	7,650,188
Unrealized Gain/ (Loss), net	(175,497)	(9,648)
Fair value	$ 7,662,038	$ 7,640,540
Period- end Weighted Average Yield	3.14%	3.09%
Quarterly Weighted Average Yield	2.96%	2.90%
Agency RMBS | ARM
Schedule of Available-for-sale Securities [Line Items]
Principal/ Notional Balance	$ 227,154	$ 238,486
Unamortized Premium (Discount)	1,419	1,609
Amortized Cost	228,573	240,095
Unrealized Gain/ (Loss), net	(907)	1,105
Fair value	$ 227,666	$ 241,200
Period- end Weighted Average Yield	2.59%	2.60%
Quarterly Weighted Average Yield	2.32%	2.36%
Agency RMBS | Hybrid ARM
Schedule of Available-for-sale Securities [Line Items]
Principal/ Notional Balance	$ 1,617,558	$ 1,696,148
Unamortized Premium (Discount)	24,314	26,066
Amortized Cost	1,641,872	1,722,214
Unrealized Gain/ (Loss), net	(18,334)	(2,829)
Fair value	$ 1,623,538	$ 1,719,385
Period- end Weighted Average Yield	2.54%	2.54%
Quarterly Weighted Average Yield	2.24%	2.25%
Agency-CMO
Schedule of Available-for-sale Securities [Line Items]
Principal/ Notional Balance	$ 1,146,728	$ 1,226,539
Unamortized Premium (Discount)	(880,126)	(942,290)
Amortized Cost	266,602	284,249
Unrealized Gain/ (Loss), net	(12,648)	(10,306)
Fair value	$ 253,954	$ 273,943
Period- end Weighted Average Yield	2.97%	2.91%
Quarterly Weighted Average Yield	2.51%	2.74%
CMBS
Schedule of Available-for-sale Securities [Line Items]
Principal/ Notional Balance	$ 3,899,661	$ 3,879,775
Unamortized Premium (Discount)	(703,191)	(704,097)
Amortized Cost	3,196,470	3,175,678
Unrealized Gain/ (Loss), net	(7,189)	40,739
Fair value	$ 3,189,281	$ 3,216,417
Period- end Weighted Average Yield	4.92%	4.92%
Quarterly Weighted Average Yield	4.85%	4.77%
Non-Agency RMBS
Schedule of Available-for-sale Securities [Line Items]
Principal/ Notional Balance	$ 2,672,496	$ 2,785,704
Unamortized Premium (Discount)	(1,606,280)	(1,661,683)
Amortized Cost	1,066,216	1,124,021
Unrealized Gain/ (Loss), net	128,736	133,587
Fair value	$ 1,194,952	$ 1,257,608
Period- end Weighted Average Yield	7.19%	7.19%
Quarterly Weighted Average Yield	7.08%	7.18%
Non-Agency RMBS | Variable Rate
Schedule of Available-for-sale Securities [Line Items]
Percentage of non-agency securities classified as variable rate	51.80%	52.20%
Non-Agency RMBS | Fixed Rate
Schedule of Available-for-sale Securities [Line Items]
Percentage of non-agency securities classified as fixed rate	38.00%	37.80%
Non-Agency RMBS | Floating Rate
Schedule of Available-for-sale Securities [Line Items]
Percentage of non-agency securities classified as floating rate	10.10%	10.00%
GSE CRT
Schedule of Available-for-sale Securities [Line Items]
Principal/ Notional Balance	$ 750,873	$ 757,183
Unamortized Premium (Discount)	23,609	24,306
Amortized Cost	774,482	781,489
Unrealized Gain/ (Loss), net	86,771	85,390
Fair value	$ 861,253	$ 866,879
Period- end Weighted Average Yield	2.61%	2.45%
Quarterly Weighted Average Yield	3.00%	2.79%