Fair Value of Assets and Liabilities - Additional Information about Servicing Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Servicing Assets, Changes in fair value due to:
Fair value at beginning of period			$ 33,676
Fair value at end of period	$ 60,160		60,160
Servicing Liabilities, Changes in fair value due to:
Fair value at beginning of period			833
Fair value at end of period	156		156
Fair Value, Measurements, Recurring
Servicing Assets, Changes in fair value due to:
Fair value at beginning of period	50,984	$ 25,901	33,676	$ 21,398
Issuances	16,308	10,563	44,476	24,460
Changes in fair value, included in servicing fees	(9,194)	(6,961)	(22,253)	(16,083)
Other net changes included in deferred revenue	2,062	118	4,261	(154)
Fair value at end of period	60,160	29,621	60,160	29,621
Servicing Liabilities, Changes in fair value due to:
Fair value at beginning of period	276	1,711	833	2,846
Issuances	0	19	0	332
Change in fair value, included in investor fees	(120)	(499)	(677)	(1,947)
Other net changes included in deferred revenue	0	0	0	0
Fair value at end of period	156	1,231	156	1,231
Asset-backed subordinated residual certificates related to Company-sponsored securitizations and CLUB Certificate transactions
Servicing Liabilities, Changes in fair value due to:
Fair value at beginning of period	28,727	3,567	10,029	0
Additions	16,836	2,372	41,641	5,939
Redemptions	(417)	0	(2,742)	0
Cash received	(3,075)	0	(4,362)	0
Change in unrealized gain (loss)	718	(31)	347	(31)
Other-than-temporary impairment	(165)	(505)	(2,289)	(505)
Fair value at end of period	$ 42,624	$ 5,403	$ 42,624	$ 5,403

Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Charge-offs

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Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Retirements

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Amount of gain (loss) recognized on the income statement for financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of purchases of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of settlements of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Fair value of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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The value of new servicing assets, subsequently measured at fair value, acquired or created during the current period through purchases or from transfers of financial assets.

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Fair value of an asset representing net future revenue from contractually specified servicing fees, late charges, and other ancillary revenues, in excess of future costs related to servicing arrangements.

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Amount of increase (decrease) in fair value from changes in the inputs, assumptions, or model used to calculate the fair value of the contract to service financial assets under which the benefits of servicing are expected to more than adequately compensate the servicer.

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Amount of increase (decrease) in fair value from changes classified as other for a contract to service financial assets under which the benefits of servicing are expected to more than adequately compensate the servicer.

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The value of new servicing liabilities, that are subsequently measured at fair value, acquired or created during the current period through assumptions of servicing obligations and servicing obligations that result from transfers of financial assets.

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Fair value of a liability representing servicing arrangements under which the estimated future revenue from contractually specified servicing fees, late charges, and other ancillary revenues are not expected to adequately compensate the servicer.

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Amount of increase (decrease) in fair value from changes classified as other for a contract to service financial assets under which the estimated future revenues from contractually specified servicing fees, late charges, and other ancillary revenues are not expected to adequately compensate the servicer.

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