Fair Value of Assets and Liabilities - Loans, Loan Servicing Rights, Related Notes and Certificates (Detail) - USD ($) $ in Thousands		Jun. 30, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held for investment	[1]	$ 2,358,629	$ 2,932,325
Loans held for investment by the Company	[1]	9,621	361,230
Loans held for sale by the Company	[1]	515,307	235,825
Securities available for sale		149,804	117,573
Servicing assets		50,984	33,676
Total assets		3,084,345	3,680,629
Notes, certificates and secured borrowings	[1]	2,377,080	2,954,768
Payable to securitization residual certificate holders		0	1,479
Loan trailing fee liability		276	833
Loan trailing fee liability		9,388	8,432
Total liabilities		2,386,744	2,965,512
Asset-backed senior securities related to Company-sponsored securitizations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale		64,987	37,020
Certificates of deposit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale		15,796	24,758
Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale		17,746	16,258
Asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale		13,075	14,843
Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale		9,473	14,665
Asset-backed subordinated residual certificates related to Company-sponsored securitizations and CLUB Certificate transactions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale		28,727	10,029
Level 1 Inputs
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held for investment		0	0
Loans held for investment by the Company		0	0
Loans held for sale by the Company		0	0
Securities available for sale		0	0
Servicing assets		0	0
Total assets		0	0
Notes, certificates and secured borrowings		0	0
Payable to securitization residual certificate holders			0
Loan trailing fee liability		0	0
Loan trailing fee liability		0	0
Total liabilities		0	0
Level 1 Inputs | Asset-backed senior securities related to Company-sponsored securitizations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale		0	0
Level 1 Inputs | Certificates of deposit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale		0	0
Level 1 Inputs | Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale		0	0
Level 1 Inputs | Asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale		0	0
Level 1 Inputs | Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale		0	0
Level 1 Inputs | Asset-backed subordinated residual certificates related to Company-sponsored securitizations and CLUB Certificate transactions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale		0	0
Level 2 Inputs
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held for investment		0	0
Loans held for investment by the Company		0	0
Loans held for sale by the Company		0	0
Securities available for sale		121,077	107,544
Servicing assets		0	0
Total assets		121,077	107,544
Notes, certificates and secured borrowings		0	0
Payable to securitization residual certificate holders			0
Loan trailing fee liability		0	0
Loan trailing fee liability		0	0
Total liabilities		0	0
Level 2 Inputs | Asset-backed senior securities related to Company-sponsored securitizations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale		64,987	37,020
Level 2 Inputs | Certificates of deposit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale		15,796	24,758
Level 2 Inputs | Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale		17,746	16,258
Level 2 Inputs | Asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale		13,075	14,843
Level 2 Inputs | Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale		9,473	14,665
Level 2 Inputs | Asset-backed subordinated residual certificates related to Company-sponsored securitizations and CLUB Certificate transactions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale		0	0
Level 3 Inputs
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held for investment		2,358,629	2,932,325
Loans held for investment by the Company		9,621	361,230
Loans held for sale by the Company		515,307	235,825
Securities available for sale		28,727	10,029
Servicing assets		50,984	33,676
Total assets		2,963,268	3,573,085
Notes, certificates and secured borrowings		2,377,080	2,954,768
Payable to securitization residual certificate holders			1,479
Loan trailing fee liability		276	833
Loan trailing fee liability		9,388	8,432
Total liabilities		2,386,744	2,965,512
Level 3 Inputs | Asset-backed senior securities related to Company-sponsored securitizations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale		0	0
Level 3 Inputs | Certificates of deposit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale		0	0
Level 3 Inputs | Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale		0	0
Level 3 Inputs | Asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale		0	0
Level 3 Inputs | Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale		0	0
Level 3 Inputs | Asset-backed subordinated residual certificates related to Company-sponsored securitizations and CLUB Certificate transactions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale		$ 28,727	$ 10,029

[1] Includes amounts in consolidated variable interest entities (VIEs) presented separately in the table below.