Fair Value - Fair Values and Related Principal Amounts Due upon Maturity of Loans Accounted for Under Fair Value Option (Detail) - USD ($) $ in Thousands	Sep. 30, 2024	Dec. 31, 2023
Loans, fair value [Abstract]
Fair value, Total	$ 1,429,525	$ 1,433,820
Loans, principal amount due upon maturity [Abstract]
Principal amount due upon maturity	1,627,149	1,704,184
Loans, difference, [Abstract]
Difference	(197,624)	(270,364)
Loans acquired for sale at fair value [Member]
Loans, fair value [Abstract]
Current through 89 days delinquent	1,665,099	667,857
90 or more days delinquent, not in foreclosure	622	433
90 or more days delinquent, in foreclosure	75	728
Fair value, Total	1,665,796	669,018
Loans, principal amount due upon maturity [Abstract]
Current through 89 days delinquent	1,619,369	648,283
90 or more days delinquent, not in foreclosure	873	617
90 or more days delinquent, in foreclosure	96	845
Principal amount due upon maturity	1,620,338	649,745
Loans, difference, [Abstract]
Current through 89 days delinquent	45,730	19,574
90 or more days delinquent, not in foreclosure	(251)	(184)
90 or more days delinquent, in foreclosure	(21)	(117)
Difference	45,458	19,273
Loans acquired for sale at fair value [Member] | Nonperforming mortgage loans [Member]
Loans, fair value [Abstract]
Fair value	697	1,161
Loans, principal amount due upon maturity [Abstract]
Principal amount due upon maturity	969	1,462
Loans, difference, [Abstract]
Difference	(272)	(301)
Loans at Fair Value Held in Consolidated VIE [Member]
Loans, fair value [Abstract]
Current through 89 days delinquent	1,426,511	1,430,427
90 or more days delinquent, not in foreclosure	891	1,262
90 or more days delinquent, in foreclosure	173	0
Fair value	1,427,575	1,431,689
Loans, principal amount due upon maturity [Abstract]
Current through 89 days delinquent	1,620,842	1,697,305
90 or more days delinquent, not in foreclosure	1,122	1,582
90 or more days delinquent, in foreclosure	219	0
Principal amount due upon maturity	1,622,183	1,698,887
Loans, difference, [Abstract]
Current through 89 days delinquent	(194,331)	(266,878)
90 or more days delinquent, not in foreclosure	(231)	(320)
90 or more days delinquent, in foreclosure	(46)	0
Difference	(194,608)	(267,198)
Loans at Fair Value Held in Consolidated VIE [Member] | Nonperforming mortgage loans [Member]
Loans, fair value [Abstract]
Fair value	1,064	1,262
Loans, principal amount due upon maturity [Abstract]
Principal amount due upon maturity	1,341	1,582
Loans, difference, [Abstract]
Difference	(277)	(320)
Distressed at Fair Value [Member]
Loans, fair value [Abstract]
Current through 89 days delinquent	460	569
90 or more days delinquent, not in foreclosure	1,490	393
90 or more days delinquent, in foreclosure	0	1,169
Fair value	1,950	2,131
Loans, principal amount due upon maturity [Abstract]
Current through 89 days delinquent	634	728
90 or more days delinquent, not in foreclosure	4,332	2,023
90 or more days delinquent, in foreclosure	0	2,546
Principal amount due upon maturity	4,966	5,297
Loans, difference, [Abstract]
Current through 89 days delinquent	(174)	(159)
90 or more days delinquent, not in foreclosure	(2,842)	(1,630)
90 or more days delinquent, in foreclosure	0	(1,377)
Difference	(3,016)	(3,166)
Distressed at Fair Value [Member] | Nonperforming mortgage loans [Member]
Loans, fair value [Abstract]
Fair value	1,490	1,562
Loans, principal amount due upon maturity [Abstract]
Principal amount due upon maturity	4,332	4,569
Loans, difference, [Abstract]
Difference	$ (2,842)	$ (3,007)