Derivative and Credit Risk Transfer Strip Assets and Liabilities - Summary of Derivative Liabilities, Financial Liabilities and Collateral Pledged by Counterparty (Detail) - Unpaid Principal Balance Before Unamortized Debt Issuance Costs Adjustment [Member] - USD ($) $ in Thousands	Sep. 30, 2024	Dec. 31, 2023
Offsetting Liabilities [Line Items]
Net amount of liabilities presented in the consolidated balance sheet	$ 5,754,195	$ 5,632,496
Gross amounts not offset in the consolidated balance sheet, Financial instruments	(5,751,519)	(5,627,807)
Gross amounts not offset in the consolidated balance sheet, Cash collateral pledged	0	0
Net amount	2,676	4,689
Interest Rate Lock Commitments [Member] | Security Sold Under Agreements to Repurchase [Member]
Offsetting Liabilities [Line Items]
Net amount of liabilities presented in the consolidated balance sheet	1,574	64
Gross amounts not offset in the consolidated balance sheet, Financial instruments	0	0
Gross amounts not offset in the consolidated balance sheet, Cash collateral pledged	0	0
Net amount	1,574	64
J.P. Morgan Securities LLC [Member] | Security Sold Under Agreements to Repurchase [Member]
Offsetting Liabilities [Line Items]
Net amount of liabilities presented in the consolidated balance sheet	1,631,801	1,521,072
Gross amounts not offset in the consolidated balance sheet, Financial instruments	(1,631,801)	(1,521,072)
Gross amounts not offset in the consolidated balance sheet, Cash collateral pledged	0	0
Net amount	0	0
Bank of America, N.A. [Member] | Security Sold Under Agreements to Repurchase [Member]
Offsetting Liabilities [Line Items]
Net amount of liabilities presented in the consolidated balance sheet	809,476	785,756
Gross amounts not offset in the consolidated balance sheet, Financial instruments	(809,476)	(785,756)
Gross amounts not offset in the consolidated balance sheet, Cash collateral pledged	0	0
Net amount	0	0
Wells Fargo Securities L L C | Security Sold Under Agreements to Repurchase [Member]
Offsetting Liabilities [Line Items]
Net amount of liabilities presented in the consolidated balance sheet	803,927	569,129
Gross amounts not offset in the consolidated balance sheet, Financial instruments	(803,927)	(569,129)
Gross amounts not offset in the consolidated balance sheet, Cash collateral pledged	0	0
Net amount	0	0
Barclays Capital Inc. [Member] | Security Sold Under Agreements to Repurchase [Member]
Offsetting Liabilities [Line Items]
Net amount of liabilities presented in the consolidated balance sheet	578,798	807,404
Gross amounts not offset in the consolidated balance sheet, Financial instruments	(578,798)	(803,641)
Gross amounts not offset in the consolidated balance sheet, Cash collateral pledged	0	0
Net amount	0	3,763
Santander US Capital [Member] | Security Sold Under Agreements to Repurchase [Member]
Offsetting Liabilities [Line Items]
Net amount of liabilities presented in the consolidated balance sheet	362,604	292,091
Gross amounts not offset in the consolidated balance sheet, Financial instruments	(362,539)	(292,091)
Gross amounts not offset in the consolidated balance sheet, Cash collateral pledged	0	0
Net amount	65	0
Citigroup Global Markets Inc. [Member] | Security Sold Under Agreements to Repurchase [Member]
Offsetting Liabilities [Line Items]
Net amount of liabilities presented in the consolidated balance sheet	310,707	147,093
Gross amounts not offset in the consolidated balance sheet, Financial instruments	(310,707)	(147,093)
Gross amounts not offset in the consolidated balance sheet, Cash collateral pledged	0	0
Net amount	0	0
Daiwa Capital Markets [Member] | Security Sold Under Agreements to Repurchase [Member]
Offsetting Liabilities [Line Items]
Net amount of liabilities presented in the consolidated balance sheet	230,033	340,975
Gross amounts not offset in the consolidated balance sheet, Financial instruments	(230,033)	(340,975)
Gross amounts not offset in the consolidated balance sheet, Cash collateral pledged	0	0
Net amount	0	0
Goldman Sachs & Co. LLC [Member]
Offsetting Liabilities [Line Items]
Net amount of liabilities presented in the consolidated balance sheet	227,545	145,007
Gross amounts not offset in the consolidated balance sheet, Financial instruments	(227,545)	(145,007)
Gross amounts not offset in the consolidated balance sheet, Cash collateral pledged	0	0
Net amount	0	0
RBC Capital Markets, L.P. [Member] | Security Sold Under Agreements to Repurchase [Member]
Offsetting Liabilities [Line Items]
Net amount of liabilities presented in the consolidated balance sheet	212,560	128,602
Gross amounts not offset in the consolidated balance sheet, Financial instruments	(212,560)	(128,602)
Gross amounts not offset in the consolidated balance sheet, Cash collateral pledged	0	0
Net amount	0	0
Atlas Securitized Products, L.P. [Member] | Security Sold Under Agreements to Repurchase [Member]
Offsetting Liabilities [Line Items]
Net amount of liabilities presented in the consolidated balance sheet	206,873	783,456
Gross amounts not offset in the consolidated balance sheet, Financial instruments	(206,873)	(783,456)
Gross amounts not offset in the consolidated balance sheet, Cash collateral pledged	0	0
Net amount	0	0
Morgan Stanley & Co. LLC [Member] | Security Sold Under Agreements to Repurchase [Member]
Offsetting Liabilities [Line Items]
Net amount of liabilities presented in the consolidated balance sheet	201,862	25,814
Gross amounts not offset in the consolidated balance sheet, Financial instruments	(201,862)	(25,814)
Gross amounts not offset in the consolidated balance sheet, Cash collateral pledged	0	0
Net amount	0	0
Mizuho Financial Group [Member] | Security Sold Under Agreements to Repurchase [Member]
Offsetting Liabilities [Line Items]
Net amount of liabilities presented in the consolidated balance sheet	98,686	67,637
Gross amounts not offset in the consolidated balance sheet, Financial instruments	(98,121)	(67,110)
Gross amounts not offset in the consolidated balance sheet, Cash collateral pledged	0	0
Net amount	565	527
BNP Paribas [Member] | Security Sold Under Agreements to Repurchase [Member]
Offsetting Liabilities [Line Items]
Net amount of liabilities presented in the consolidated balance sheet	77,277	10,121
Gross amounts not offset in the consolidated balance sheet, Financial instruments	(77,277)	(10,121)
Gross amounts not offset in the consolidated balance sheet, Cash collateral pledged	0	0
Net amount	0	0
Nomura Holdings America, Inc [Member]
Offsetting Liabilities [Line Items]
Net amount of liabilities presented in the consolidated balance sheet	0	8,135
Gross amounts not offset in the consolidated balance sheet, Financial instruments	0	(7,940)
Gross amounts not offset in the consolidated balance sheet, Cash collateral pledged	0	0
Net amount	0	195
Other [Member] | Security Sold Under Agreements to Repurchase [Member]
Offsetting Liabilities [Line Items]
Net amount of liabilities presented in the consolidated balance sheet	472	140
Gross amounts not offset in the consolidated balance sheet, Financial instruments	0	0
Gross amounts not offset in the consolidated balance sheet, Cash collateral pledged	0	0
Net amount	$ 472	$ 140