Mortgage-Backed Securities - Maturities (Detail) - USD ($)	Mar. 31, 2020	Dec. 31, 2019
Fair value
Less than one year, at fair value	$ 27,145,000	$ 31,408,000
Greater than one year and less than five years, at fair value	529,004,000	6,582,162,000
Greater than five years and less than ten years, at fair value	920,185,000	1,536,886,000
Greater than ten years, at fair value	3,636,426,000	3,804,962,000
Total maturities, at fair value	5,112,760,000	11,955,418,000
Amortized cost
Less than one year, at amortized cost	29,223,000	30,290,000
Greater than one year and less than five years, at amortized cost	482,936,000	6,305,410,000
Greater than five years and less than ten years, at amortized cost	783,032,000	1,253,782,000
Greater than ten years, at amortized cost	3,295,212,000	3,409,872,000
Amortized Cost	4,590,403,000	10,999,354,000
Non-Agency RMBS - Senior
Amortized cost
Amortized Cost	165,966,737
Non-Agency RMBS - Subordinated
Amortized cost
Amortized Cost	55,410,172
Residential Mortgage-Backed Securities | Non-Agency RMBS - Senior
Fair value
Less than one year, at fair value	9,861,000	16,343,000
Greater than one year and less than five years, at fair value	344,239,000	450,185,000
Greater than five years and less than ten years, at fair value	559,033,000	676,382,000
Greater than ten years, at fair value	507,091,000	558,001,000
Total maturities, at fair value	1,420,224,000	1,700,911,000
Amortized cost
Less than one year, at amortized cost	9,961,000	15,206,000
Greater than one year and less than five years, at amortized cost	273,374,000	304,850,000
Greater than five years and less than ten years, at amortized cost	369,124,000	409,958,000
Greater than ten years, at amortized cost	350,848,000	342,672,000
Amortized Cost	1,003,307,000	1,072,686,000
Residential Mortgage-Backed Securities | Non-Agency RMBS - Subordinated
Fair value
Less than one year, at fair value	0	0
Greater than one year and less than five years, at fair value	29,670,000	43,796,000
Greater than five years and less than ten years, at fair value	112,528,000	95,973,000
Greater than ten years, at fair value	385,980,000	484,829,000
Total maturities, at fair value	528,178,000	624,598,000
Amortized cost
Less than one year, at amortized cost	0	0
Greater than one year and less than five years, at amortized cost	21,487,000	29,085,000
Greater than five years and less than ten years, at amortized cost	111,344,000	86,033,000
Greater than ten years, at amortized cost	434,173,000	438,476,000
Amortized Cost	567,004,000	553,594,000
Residential Mortgage-Backed Securities | Non-Agency RMBS - Senior interest-only
Fair value
Less than one year, at fair value	0	0
Greater than one year and less than five years, at fair value	113,900,000	126,631,000
Greater than five years and less than ten years, at fair value	109,067,000	159,057,000
Greater than ten years, at fair value	3,031,000	3,211,000
Total maturities, at fair value	225,998,000	288,899,000
Amortized cost
Less than one year, at amortized cost	1,752,000	0
Greater than one year and less than five years, at amortized cost	145,034,000	150,221,000
Greater than five years and less than ten years, at amortized cost	137,145,000	148,889,000
Greater than ten years, at amortized cost	3,161,000	2,060,000
Amortized Cost	287,092,000	301,170,000
Residential Mortgage-Backed Securities | Agency MBS - Residential
Fair value
Less than one year, at fair value	0	0
Greater than one year and less than five years, at fair value	0	5,939,408,000
Greater than five years and less than ten years, at fair value	0	421,539,000
Greater than ten years, at fair value	0	1,679,000
Total maturities, at fair value	0	6,362,626,000
Amortized cost
Less than one year, at amortized cost	0	0
Greater than one year and less than five years, at amortized cost	0	5,796,044,000
Greater than five years and less than ten years, at amortized cost	0	414,482,000
Greater than ten years, at amortized cost	0	1,044,000
Amortized Cost	0	6,211,570,000
Residential Mortgage-Backed Securities | Agency RMBS - Interest-only
Fair value
Less than one year, at fair value	0	0
Greater than one year and less than five years, at fair value	1,250,000	1,614,000
Greater than five years and less than ten years, at fair value	104,394,000	126,053,000
Greater than ten years, at fair value	0	0
Total maturities, at fair value	105,644,000	127,667,000
Amortized cost
Less than one year, at amortized cost	0	0
Greater than one year and less than five years, at amortized cost	2,162,000	2,260,000
Greater than five years and less than ten years, at amortized cost	133,018,000	137,276,000
Greater than ten years, at amortized cost	0	0
Amortized Cost	135,180,000	139,536,000
Commercial Mortgage Backed Securities | Agency MBS - Commercial
Fair value
Less than one year, at fair value	14,951,000	15,065,000
Greater than one year and less than five years, at fair value	0	0
Greater than five years and less than ten years, at fair value	31,252,000	29,385,000
Greater than ten years, at fair value	2,740,324,000	2,757,242,000
Total maturities, at fair value	2,786,527,000	2,801,692,000
Amortized cost
Less than one year, at amortized cost	14,981,000	15,084,000
Greater than one year and less than five years, at amortized cost	0	0
Greater than five years and less than ten years, at amortized cost	28,646,000	28,954,000
Greater than ten years, at amortized cost	2,507,030,000	2,625,620,000
Amortized Cost	2,550,657,000	2,669,658,000
Commercial Mortgage Backed Securities | Agency CMBS - Interest-only
Fair value
Less than one year, at fair value	2,333,000	0
Greater than one year and less than five years, at fair value	39,945,000	20,528,000
Greater than five years and less than ten years, at fair value	3,911,000	28,497,000
Greater than ten years, at fair value	0	0
Total maturities, at fair value	46,189,000	49,025,000
Amortized cost
Less than one year, at amortized cost	2,529,000	0
Greater than one year and less than five years, at amortized cost	40,879,000	22,950,000
Greater than five years and less than ten years, at amortized cost	3,755,000	28,190,000
Greater than ten years, at amortized cost	0	0
Amortized Cost	$ 47,163,000	$ 51,140,000