Mortgage-Backed Securities - Maturities (Detail) - USD ($)	Jun. 30, 2020	Dec. 31, 2019
Fair value
Less than one year, at fair value	$ 16,626,000	$ 31,408,000
Greater than one year and less than five years, at fair value	586,430,000	6,582,162,000
Greater than five years and less than ten years, at fair value	914,188,000	1,536,886,000
Greater than ten years, at fair value	3,386,533,000	3,804,962,000
Total maturities, at fair value	4,903,777,000	11,955,418,000
Amortized cost
Less than one year, at amortized cost	17,558,000	30,290,000
Greater than one year and less than five years, at amortized cost	471,806,000	6,305,410,000
Greater than five years and less than ten years, at amortized cost	688,941,000	1,253,782,000
Greater than ten years, at amortized cost	3,045,962,000	3,409,872,000
Amortized Cost	4,224,267,000	10,999,354,000
Non-Agency RMBS - Senior
Amortized cost
Amortized Cost	80,221,558
Non-Agency RMBS - Subordinated
Amortized cost
Amortized Cost	55,859,406
Residential Mortgage-Backed Securities | Non-Agency RMBS - Senior
Fair value
Less than one year, at fair value	0	16,343,000
Greater than one year and less than five years, at fair value	363,282,000	450,185,000
Greater than five years and less than ten years, at fair value	514,141,000	676,382,000
Greater than ten years, at fair value	464,391,000	558,001,000
Total maturities, at fair value	1,341,814,000	1,700,911,000
Amortized cost
Less than one year, at amortized cost	0	15,206,000
Greater than one year and less than five years, at amortized cost	260,682,000	304,850,000
Greater than five years and less than ten years, at amortized cost	313,117,000	409,958,000
Greater than ten years, at amortized cost	305,632,000	342,672,000
Amortized Cost	879,431,000	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	37,567,000	43,796,000
Greater than five years and less than ten years, at fair value	118,921,000	95,973,000
Greater than ten years, at fair value	366,167,000	484,829,000
Total maturities, at fair value	522,655,000	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	23,158,000	29,085,000
Greater than five years and less than ten years, at amortized cost	119,819,000	86,033,000
Greater than ten years, at amortized cost	419,029,000	438,476,000
Amortized Cost	562,006,000	553,594,000
Residential Mortgage-Backed Securities | Non-Agency RMBS - Senior interest-only
Fair value
Less than one year, at fair value	234,000	0
Greater than one year and less than five years, at fair value	149,778,000	126,631,000
Greater than five years and less than ten years, at fair value	167,634,000	159,057,000
Greater than ten years, at fair value	2,055,000	3,211,000
Total maturities, at fair value	319,701,000	288,899,000
Amortized cost
Less than one year, at amortized cost	979,000	0
Greater than one year and less than five years, at amortized cost	150,770,000	150,221,000
Greater than five years and less than ten years, at amortized cost	120,614,000	148,889,000
Greater than ten years, at amortized cost	2,662,000	2,060,000
Amortized Cost	275,025,000	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	1,176,000	1,614,000
Greater than five years and less than ten years, at fair value	106,087,000	126,053,000
Greater than ten years, at fair value	0	0
Total maturities, at fair value	107,263,000	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	2,067,000	2,260,000
Greater than five years and less than ten years, at amortized cost	128,543,000	137,276,000
Greater than ten years, at amortized cost	0	0
Amortized Cost	130,610,000	139,536,000
Commercial Mortgage Backed Securities | Agency MBS - Commercial
Fair value
Less than one year, at fair value	14,862,000	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	3,547,000	29,385,000
Greater than ten years, at fair value	2,553,920,000	2,757,242,000
Total maturities, at fair value	2,572,329,000	2,801,692,000
Amortized cost
Less than one year, at amortized cost	14,877,000	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	3,175,000	28,954,000
Greater than ten years, at amortized cost	2,318,639,000	2,625,620,000
Amortized Cost	2,336,691,000	2,669,658,000
Commercial Mortgage Backed Securities | Agency CMBS - Interest-only
Fair value
Less than one year, at fair value	1,530,000	0
Greater than one year and less than five years, at fair value	34,627,000	20,528,000
Greater than five years and less than ten years, at fair value	3,858,000	28,497,000
Greater than ten years, at fair value	0	0
Total maturities, at fair value	40,015,000	49,025,000
Amortized cost
Less than one year, at amortized cost	1,702,000	0
Greater than one year and less than five years, at amortized cost	35,129,000	22,950,000
Greater than five years and less than ten years, at amortized cost	3,673,000	28,190,000
Greater than ten years, at amortized cost	0	0
Amortized Cost	$ 40,504,000	$ 51,140,000