Variable Interest Entities - Summary of Credit Risk Transfer Agreements (Detail) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2030	Sep. 30, 2029	Sep. 30, 2028	Sep. 30, 2027	Sep. 30, 2026	Sep. 30, 2025	Dec. 31, 2023
CRT strips:
Net investment income		$ 80,864	$ 163,429	$ 226,267	$ 344,247
Carrying value of CRT arrangements:
Derivative and credit risk transfer strip assets		81,844		81,844								$ 177,984
Interest-only security payable at fair value		35,098		35,098		$ 35,098	$ 0	$ 0	$ 0	$ 0	$ 0	32,667
Variable Interest Entities [Member]
Carrying value of CRT arrangements:
Interest-only security payable at fair value		35,098		35,098								32,667
Variable Interest Entities [Member] | Credit Risk Transfer Agreements [Member]
CRT derivatives:
Realized		3,275	4,051	10,248	12,504
Valuation changes		5,460	9,113	13,716	26,619
Gains recognized on gross derivative related to credit risk transactions		8,735	13,164	23,964	39,123
CRT strips:
Realized		10,990	11,241	34,368	35,529
Valuation changes		3,499	9,977	33,217	68,601
Credit risk transfer strips		14,489	21,218	67,585	104,130
Interest-only security payable at fair value		(2,390)	(4,228)	(2,431)	(6,363)
Gains recognized on gross derivative related to credit risk transactions		20,834	30,154	89,118	136,890
Interest income — Deposits securing CRT arrangements		15,042	16,419	46,121	46,410
Net investment income		35,876	46,573	135,239	183,300
Net payments made to settle losses on CRT arrangements		827	$ 496	1,140	$ 2,252
Carrying value of CRT arrangements:
Derivative assets - CRT derivatives		29,690		29,690								16,160
CRT strip liabilities		(13,475)		(13,475)								(46,692)
Derivative and credit risk transfer strip assets		1,116,564		1,116,564								1,146,299
Deposits securing CRT arrangements		1,135,447		1,135,447								1,209,498
Interest-only security payable at fair value		(35,098)		(35,098)								(32,667)
Derivative assets		29,690		29,690								16,160
Derivative and credit risk transfer strip assets pledged to secure	[1]	1,135,447		1,135,447								1,209,498
UPB of loans underlying CRT arrangements		21,708,165		21,708,165								23,152,230
Variable Interest Entities [Member] | Credit Risk Transfer Agreements [Member] | Purchase Commitment [Member]
Collection status (UPB):
Current		21,105,679		21,105,679								22,531,905
30-89 days delinquent		424,102		424,102								411,991
90-180 days delinquent		119,236		119,236								120,011
180 or more days delinquent		42,283		42,283								64,647
Foreclosure		16,865		16,865								23,676
Bankruptcy		$ 64,331		$ 64,331								$ 58,696

[1] Deposits securing credit risk transfer arrangements also secure $13.5 million and $46.7 million in CRT strip liabilities at September 30, 2024 and December 31, 2023, respectively.