Fair Value of Assets and Liabilities - Loans, Loan Servicing Rights, Related Notes and Certificates (Detail) - USD ($) $ in Thousands		Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held for investment	[1],[2]	$ 2,932,325	$ 4,295,121
Loans held for investment by the Company	[1],[2]	361,230	16,863
Loans held for sale by the Company at fair value (1)	[1]	235,825	9,048
Securities available for sale		117,573	287,137
Servicing assets		33,676	21,398
Total assets		3,680,629	4,629,567
Notes, certificates and secured borrowings	[1]	2,954,768	4,320,895
Payable to securitization residual certificate holders		1,479	0
Servicing liabilities		833	2,846
Loan trailing fee liability		8,432	4,913
Total liabilities		2,965,512	4,328,654
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 at fair value (1)		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
Servicing liabilities		0	0
Loan trailing fee liability		0	0
Total liabilities		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 at fair value (1)		0	0
Securities available for sale		107,544	287,137
Servicing assets		0	0
Total assets		107,544	287,137
Notes, certificates and secured borrowings		0	0
Payable to securitization residual certificate holders		0
Servicing liabilities		0	0
Loan trailing fee liability		0	0
Total liabilities		0	0
Level 3 Inputs
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held for investment		2,932,325	4,295,121
Loans held for investment by the Company		361,230	16,863
Loans held for sale by the Company at fair value (1)		235,825	9,048
Securities available for sale		10,029	0
Servicing assets		33,676	21,398
Total assets		3,573,085	4,342,430
Notes, certificates and secured borrowings		2,954,768	4,320,895
Payable to securitization residual certificate holders		1,479
Servicing liabilities		833	2,846
Loan trailing fee liability		8,432	4,913
Total liabilities		2,965,512	4,328,654
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		37,020
Asset-backed senior securities related to Company-sponsored securitizations | Level 1 Inputs
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale		0
Asset-backed senior securities related to Company-sponsored securitizations | Level 2 Inputs
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale		37,020
Asset-backed senior securities related to Company-sponsored securitizations | Level 3 Inputs
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale		0
Certificates of deposit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale		24,758	27,501
Certificates of deposit | Level 1 Inputs
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale		0	0
Certificates of deposit | Level 2 Inputs
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale		24,758	27,501
Certificates of deposit | Level 3 Inputs
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale		0	0
Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale		16,258	181,223
Corporate debt securities | Level 1 Inputs
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale		0	0
Corporate debt securities | Level 2 Inputs
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale		16,258	181,223
Corporate debt securities | Level 3 Inputs
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale		0	0
Asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale		14,843	25,364
Asset-backed securities | Level 1 Inputs
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale		0	0
Asset-backed securities | Level 2 Inputs
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale		14,843	25,364
Asset-backed securities | Level 3 Inputs
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale		0	0
Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale		14,665	20,164
Commercial paper | Level 1 Inputs
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale		0	0
Commercial paper | Level 2 Inputs
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale		14,665	20,164
Commercial paper | Level 3 Inputs
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale		0	0
U.S. agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale			19,623
U.S. agency securities | Level 1 Inputs
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale			0
U.S. agency securities | Level 2 Inputs
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale			19,623
U.S. agency securities | Level 3 Inputs
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale			0
U.S. Treasury securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale			2,496
U.S. Treasury securities | Level 1 Inputs
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale			0
U.S. Treasury securities | Level 2 Inputs
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale			2,496
U.S. Treasury securities | Level 3 Inputs
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale			0
Other securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale			10,766
Other securities | Level 1 Inputs
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale			0
Other securities | Level 2 Inputs
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale			10,766
Other securities | Level 3 Inputs
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale			$ 0
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		10,029
Asset-backed subordinated residual certificates related to Company-sponsored securitizations and CLUB Certificate transactions | Level 1 Inputs
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale		0
Asset-backed subordinated residual certificates related to Company-sponsored securitizations and CLUB Certificate transactions | Level 2 Inputs
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale		0
Asset-backed subordinated residual certificates related to Company-sponsored securitizations and CLUB Certificate transactions | Level 3 Inputs
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale		$ 10,029

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

[2] Prior period amounts have been reclassified to conform to the current period presentation. See “Notes to Consolidated Financial Statements – Note 1. Basis of Presentation” for additional information.