Debt - Schedule of Carrying Amounts of Consolidated VIE's Assets and Liabilities (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Jun. 30, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019
Assets
Cash	$ 8,146		$ 8,052	$ 4,292		$ 2,263
Net finance receivables	1,314,233		1,136,259	1,059,554
Allowance for credit losses	(150,100)	$ (139,400)	(150,000)	(144,000)	$ (142,000)	(62,200)
Restricted cash	103,999		63,824	$ 58,219		$ 54,164
Other assets	18,317		16,332
Total assets	1,313,558		1,103,856
Liabilities
Net long-term debt	968,693		762,248
Accounts payable and accrued expenses	36,114		40,284
Total liabilities	1,036,092		831,733
Variable Interest Entity, Primary Beneficiary [Member]
Assets
Cash	315		236
Net finance receivables	854,036		483,674
Allowance for credit losses	(92,964)		(59,046)
Restricted cash	86,297		51,849
Other assets	134		5
Total assets	847,818		476,718
Liabilities
Net long-term debt	838,711		477,822
Accounts payable and accrued expenses	108		87
Total liabilities	$ 838,819		$ 477,909