Mortgage-Backed Securities - Summary of Investments in Mortgage Backed Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2024	Dec. 31, 2023
Mortgage Backed Securities [Line Items]
Fair value	$ 4,068,337	$ 4,836,292
Mortgage Backed Securities [Member]
Mortgage Backed Securities [Line Items]
Principal balance or notional amount	4,139,851	4,777,649
Unamortized net purchase premiums (discounts)	(122,753)	(25,733)
Cumulative valuation changes	(36,602)	(9,855)
Fair value	3,980,496	4,742,061
Interest-Only Stripped Mortgage-Backed Securities [Member]
Mortgage Backed Securities [Line Items]
Principal balance or notional amount	405,346	419,791
Fair value	87,841	94,231
Mortgage-Backed Securities Including Interest-Only Stripped Mortgage-Backed Securities [Member]
Mortgage Backed Securities [Line Items]
Fair value	4,068,337	4,836,292
Agency Fixed Rate Pass Through Securities [Member] | Mortgage Backed Securities [Member]
Mortgage Backed Securities [Line Items]
Principal balance or notional amount	3,295,288	4,311,342
Unamortized net purchase premiums (discounts)	(1,744)	34
Cumulative valuation changes	(52,728)	(41,320)
Fair value	3,240,816	4,270,056
Principal - Only Stripped Mortgage - Backed Securities [Member] | Mortgage Backed Securities [Member]
Mortgage Backed Securities [Line Items]
Principal balance or notional amount	551,502	65,573
Unamortized net purchase premiums (discounts)	(113,872)	(18,567)
Cumulative valuation changes	(4,999)	6,330
Fair value	432,631	53,336
Subordinate Credit-Linked Securities | Mortgage Backed Securities [Member]
Mortgage Backed Securities [Line Items]
Principal balance or notional amount	174,813	275,963
Unamortized net purchase premiums (discounts)	(3,687)	(3,633)
Cumulative valuation changes	25,684	28,850
Fair value	196,810	301,180
Senior Non-Agency Securities [Member] | Mortgage Backed Securities [Member]
Mortgage Backed Securities [Line Items]
Principal balance or notional amount	118,248	124,771
Unamortized net purchase premiums (discounts)	(3,450)	(3,567)
Cumulative valuation changes	(4,559)	(3,715)
Fair value	$ 110,239	$ 117,489