Summary of Significant Accounting Policies - Additional Information (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2024 USD ($)	Sep. 30, 2023 USD ($)	Sep. 30, 2024 USD ($) Facility	Sep. 30, 2023 USD ($)	Nov. 12, 2024	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)
Accounting Policies [Line Items]
Other comprehensive income (loss)	$ 0	$ 0	$ 0	$ 0
Valuation allowance for finance receivables classified as held for sale	32,600,000		32,600,000			$ 33,800,000
Aggregate principal balance held for sale at fair value	310,800,000		310,800,000
Finance receivables held for sale at fair value	277,600,000		277,600,000			0
Aggregate principal balance held for investment at fair value	250,700,000		250,700,000
Finance receivables held for investment at fair value	210,000,000		210,000,000
Cash and cash equivalents	51,093,000		51,093,000			135,585,000	$ 398,915,000
Restricted cash	$ 47,068,000		$ 47,068,000			73,234,000	$ 73,095,000
Percentage of pro rata share	92.94%		92.94%
ASU 2021-08
Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adopted [true false]	true		true
Change in accounting principle, accounting standards update, adoption date	Jan. 01, 2023		Jan. 01, 2023
Change in accounting principle, accounting standards update, immaterial effect [true false]	true		true
Warehouse Credit Facility
Accounting Policies [Line Items]
Restricted cash	$ 15,760,000		$ 15,760,000			20,688,000
Increased borrowing capacity	32,900,000		$ 32,900,000			$ 56,900,000
0.75% Unsecured Convertible Senior Notes Due 2026
Accounting Policies [Line Items]
Redemption percentage of principal amount			100.00%
Subsequent Event
Accounting Policies [Line Items]
Percentage of pro rata share					92.94%
UACC
Accounting Policies [Line Items]
Number of warehouse credit facilities | Facility			4
UACC | Warehouse Credit Facility
Accounting Policies [Line Items]
Cash deposits included in restricted cash	46,200,000		$ 46,200,000
Aggregate borrowing limit	825,000,000		825,000,000
Increased borrowing capacity	32,900,000		32,900,000
Outstanding borrowings	$ 321,800,000		$ 321,800,000