Variable Interest Entities and Securitizations - Summary of Total Assets and Total Liabilities Associated With Variable Interests in Consolidated VIEs (Details) - USD ($) $ in Thousands	Sep. 30, 2024	Dec. 31, 2023	Dec. 31, 2022
Assets:
Restricted cash	$ 47,068	$ 73,234	$ 73,095
Finance receivables at fair value	487,573	348,670
Finance receivables held for sale, net	363,029	503,546	321,626
Interest receivable	14,024	14,484
Other assets	36,446	59,429	46,212
Total assets	1,123,734	1,475,422	1,619,027
Liabilities:
Warehouse credit facilities	321,812	421,268
Other liabilities	51,474	61,321
Total liabilities	1,119,051	1,347,751	$ 1,139,194
Securitization Vehicles
Assets:
Restricted cash	30,390	28,458
Finance receivables at fair value	244,981	316,998
Finance receivables held for sale, net	201,271
Interest receivable	7,617	6,107
Other assets	7,462	6,283
Total assets	491,721	357,846
Liabilities:
Securitization debt	410,011	314,095
Other liabilities	7,136	4,534
Total liabilities	417,147	318,629
Warehouse Facilities
Assets:
Restricted cash	15,760	20,688
Finance receivables at fair value	201,672	24,446
Finance receivables held for sale, net	152,547	457,185
Interest receivable	5,520	7,586
Other assets	4,761	6,987
Total assets	380,260	516,892
Liabilities:
Warehouse credit facilities	321,812	421,268
Other liabilities	9,227	9,801
Total liabilities	331,039	431,069
Consolidated VIEs
Assets:
Restricted cash	46,150	49,146
Finance receivables at fair value	446,653	341,444
Finance receivables held for sale, net	353,818	457,185
Interest receivable	13,137	13,693
Other assets	12,223	13,270
Total assets	871,981	874,738
Liabilities:
Securitization debt	410,011	314,095
Warehouse credit facilities	321,812	421,268
Other liabilities	16,363	14,335
Total liabilities	$ 748,186	$ 749,698