Long-Term Debt - Schedule of Carrying Amounts of Consolidated VIE's Assets and Liabilities (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Assets
Cash	$ 2,331	$ 3,657	$ 3,247	$ 5,230
Finance receivables	912,250	932,243	804,956
Allowance for credit losses	(56,400)	(58,300)	(47,750)	(48,910)
Restricted cash	38,917	46,484	$ 19,064	$ 16,787
Other assets	7,635	8,375
Total assets	953,467	956,395
Liabilities
Net long-term debt	620,448	651,349
Accounts payable and accrued expenses	17,470	25,138
Total liabilities	665,651	677,234
Variable Interest Entity, Primary Beneficiary [Member]
Assets
Cash	168	168
Finance receivables	340,396	342,481
Allowance for credit losses	(17,875)	(18,378)
Restricted cash	31,547	39,361
Other assets	110	75
Total assets	354,346	363,707
Liabilities
Net long-term debt	321,662	324,879
Accounts payable and accrued expenses	57	25
Total liabilities	$ 321,719	$ 324,904