Mortgage-Backed Securities - Summary of Investments in Mortgage Backed Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2024	Dec. 31, 2023
Mortgage Backed Securities [Line Items]
Fair value	$ 4,182,382	$ 4,836,292
Mortgage Backed Securities [Member]
Mortgage Backed Securities [Line Items]
Principal balance or notional amount	4,139,153	4,777,649
Unamortized net purchase premiums (discounts)	(128,828)	(25,733)
Cumulative valuation changes	90,801	(9,855)
Fair value	4,101,126	4,742,061
Interest-Only Stripped Mortgage-Backed Securities [Member]
Mortgage Backed Securities [Line Items]
Principal balance or notional amount	395,952	419,791
Fair value	81,256	94,231
Mortgage-Backed Securities Including Interest-Only Stripped Mortgage-Backed Securities [Member]
Mortgage Backed Securities [Line Items]
Fair value	4,182,382	4,836,292
Agency Fixed Rate Pass Through Securities [Member] | Mortgage Backed Securities [Member]
Mortgage Backed Securities [Line Items]
Principal balance or notional amount	3,217,437	4,311,342
Unamortized net purchase premiums (discounts)	(2,344)	34
Cumulative valuation changes	37,815	(41,320)
Fair value	3,252,908	4,270,056
Principal - Only Stripped Mortgage - Backed Securities [Member] | Mortgage Backed Securities [Member]
Mortgage Backed Securities [Line Items]
Principal balance or notional amount	631,601	65,573
Unamortized net purchase premiums (discounts)	(118,563)	(18,567)
Cumulative valuation changes	27,751	6,330
Fair value	540,789	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)	(4,744)	(3,633)
Cumulative valuation changes	26,242	28,850
Fair value	196,311	301,180
Senior Non-Agency Securities [Member] | Mortgage Backed Securities [Member]
Mortgage Backed Securities [Line Items]
Principal balance or notional amount	115,302	124,771
Unamortized net purchase premiums (discounts)	(3,177)	(3,567)
Cumulative valuation changes	(1,007)	(3,715)
Fair value	$ 111,118	$ 117,489