Mortgage-Backed Securities - Summary of Investments in Mortgage Backed Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2025	Dec. 31, 2024
Mortgage Backed Securities [Line Items]
Fair value	$ 3,967,045	$ 4,063,706
Mortgage Backed Securities [Member]
Mortgage Backed Securities [Line Items]
Principal balance or notional amount	4,010,311	4,194,752
Unearned purchase discounts, net	(156,351)	(169,422)
Cumulative valuation changes	34,033	(47,884)
Fair value	3,887,993	3,977,446
Total	3,967,045
Maturing after five year through ten years	41,106
Maturing after ten years	3,925,939
Interest-Only Stripped Securities [Member]
Mortgage Backed Securities [Line Items]
Principal balance or notional amount	366,767	386,040
Fair value	79,052	86,260
Total	79,052
Maturing after ten years	79,052
Mortgage-Backed Securities Including Interest-Only Stripped Mortgage-Backed Securities [Member]
Mortgage Backed Securities [Line Items]
Fair value	3,967,045	4,063,706
Agency Fixed Rate Pass Through Securities [Member] \| Mortgage Backed Securities [Member]
Mortgage Backed Securities [Line Items]
Principal balance or notional amount	2,981,885	3,132,005
Unearned purchase discounts, net	(1,988)	(901)
Cumulative valuation changes	(1,950)	(51,612)
Fair value	2,977,947	3,079,492
Total	2,977,947
Maturing after ten years	2,977,947
Principal-only Stripped Securities [Member] \| Mortgage Backed Securities [Member]
Mortgage Backed Securities [Line Items]
Principal balance or notional amount	712,819	776,455
Unearned purchase discounts, net	(146,307)	(160,960)
Cumulative valuation changes	12,000	(19,195)
Fair value	578,512	596,300
Total	578,512
Maturing after ten years	578,512
Subordinate Credit-Linked Securities \| Mortgage Backed Securities [Member]
Mortgage Backed Securities [Line Items]
Principal balance or notional amount	174,813	174,813
Unearned purchase discounts, net	(4,858)	(4,292)
Cumulative valuation changes	25,446	25,951
Fair value	195,401	196,472
Total	195,401
Maturing after five year through ten years	41,106
Maturing after ten years	154,295
Senior Non-Agency Securities [Member] \| Mortgage Backed Securities [Member]
Mortgage Backed Securities [Line Items]
Principal balance or notional amount	140,794	111,479
Unearned purchase discounts, net	(3,198)	(3,269)
Cumulative valuation changes	(1,463)	(3,028)
Fair value	136,133	$ 105,182
Total	136,133
Maturing after ten years	$ 136,133

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