Mortgage-Backed Securities - Summary of Investment in Mortgage Backed Securities Activity (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Mortgage Backed Securities [Line Items]
Balance at beginning of period			$ 4,836,292
Changes in fair value included in income arising from:
Balance at end of period	$ 4,182,382		4,182,382
Mortgage Backed Securities [Member]
Mortgage Backed Securities [Line Items]
Balance at beginning of period	4,068,337	$ 4,731,341	4,836,292	$ 4,462,601
Purchases	80,500	64,384	479,960	3,108,701
Sales	(35,667)	0	(977,007)	(2,629,540)
Repayments	(105,493)	(90,319)	(268,122)	(250,733)
Exchange of mortgage servicing spread for interest-only stripped mortgage-backed securities	35,609	103,547	35,609	103,547
Changes in fair value included in income arising from:
Accrual (amortization) of net purchase premiums and discounts	15,663	1,048	25,340	(1,172)
Valuation adjustments, net	123,433	(144,031)	50,310	(127,434)
Total changes in fair value included in income	139,096	(142,983)	75,650	(128,606)
Balance at end of period	$ 4,182,382	$ 4,665,970	$ 4,182,382	$ 4,665,970