Mortgage-Backed Securities - Summary of Investments in Mortgage Backed Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2026	Dec. 31, 2025
Mortgage Backed Securities [Line Items]
Fair value	$ 4,305,991	$ 4,452,859
Mortgage Backed Securities [Member]
Mortgage Backed Securities [Line Items]
Principal balance or notional amount	4,485,687	4,421,692
Purchase premiums (discounts), net	(121,713)	(121,798)
Cumulative valuation changes	(133,247)	80,463
Fair value	4,230,727	4,380,357
Total	4,305,991
Maturing after one year through five years	3,926
Maturing after ten years	4,302,065
Interest-Only Stripped Securities [Member]
Mortgage Backed Securities [Line Items]
Principal balance or notional amount	310,930	344,592
Fair value	75,264	72,502
Total	75,264
Maturing after one year through five years	0
Maturing after ten years	75,264
Mortgage-Backed Securities Including Interest-Only Stripped Mortgage-Backed Securities [Member]
Mortgage Backed Securities [Line Items]
Fair value	4,305,991	4,452,859
Agency Fixed Rate Pass Through [Member] \| Mortgage Backed Securities [Member]
Mortgage Backed Securities [Line Items]
Principal balance or notional amount	2,091,972	2,805,895
Purchase premiums (discounts), net	2,292	(2,125)
Cumulative valuation changes	(90,840)	46,677
Fair value	2,003,424	2,850,447
Total	2,003,424
Maturing after one year through five years	0
Maturing after ten years	2,003,424
Floating Rate Collateralized Mortgage Obligations [Member] \| Mortgage Backed Securities [Member]
Mortgage Backed Securities [Line Items]
Principal balance or notional amount	1,796,823	850,172
Purchase premiums (discounts), net	5,603	(1,249)
Cumulative valuation changes	(31,775)	7,074
Fair value	1,770,651	855,997
Total	1,770,651
Maturing after one year through five years	0
Maturing after ten years	1,770,651
Principal-only Stripped [Member] \| Mortgage Backed Securities [Member]
Mortgage Backed Securities [Line Items]
Principal balance or notional amount	465,593	610,256
Purchase premiums (discounts), net	(126,661)	(115,385)
Cumulative valuation changes	(3,742)	26,258
Fair value	335,190	521,129
Total	335,190
Maturing after one year through five years	0
Maturing after ten years	335,190
Senior Non-Agency [Member] \| Mortgage Backed Securities [Member]
Mortgage Backed Securities [Line Items]
Principal balance or notional amount	127,299	155,369
Purchase premiums (discounts), net	(2,947)	(3,039)
Cumulative valuation changes	(6,816)	454
Fair value	117,536	$ 152,784
Total	117,536
Maturing after one year through five years	0
Maturing after ten years	117,536
Subordinate Residential Transition [Member] \| Mortgage Backed Securities [Member]
Mortgage Backed Securities [Line Items]
Principal balance or notional amount	4,000
Purchase premiums (discounts), net	0
Cumulative valuation changes	(74)
Fair value	3,926
Total	3,926
Maturing after one year through five years	3,926
Maturing after ten years	$ 0

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