The standard percentage of eligible secured finance receivables that is available to be advanced from credit facility.

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The standard percentage of eligible unsecured finance receivables that is available to be advanced from credit facility.

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The current period percentage of eligible secured finance receivables that is available to be advanced from credit facility.

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The current period percentage of eligible unsecured finance receivables that is available to be advanced from credit facility.

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The additional advance on an existing loan agreement.

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The maturity date of the amortizing loan credit agreement, in CCYY-MM format.

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The threshold, expressed as the outstanding principal as a percentage of the original loan amount, at which the credit agreement allows the Company to prepay the loan.

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Debt instrument revolving period end date, in CCYY-MM format.

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The amount by which the borrowing capacity of the line of credit facility may be expanded.

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The fee, expressed as a percentage of the line of credit facility, for available but unused credit capacity under the credit facility that has been decreased when the average outstanding balance exceeds contractually-designated balances.

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Percentage points added to the reference rate to compute the variable rate on the debt instrument.

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The reference rate for the variable rate of the debt instrument, such as LIBOR or the US Treasury rate and the maturity of the reference rate used, such as three months or six months LIBOR.

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Face (par) amount of debt instrument at time of issuance.

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Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium.

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Description of the payment terms of the debt instrument (for example, whether periodic payments include principal and frequency of payments) and discussion about any contingencies associated with the payment.

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Amount of unused borrowing capacity under the long-term financing arrangement that is available to the entity as of the balance sheet date.

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Weighted average interest rate of debt outstanding.

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Average amount borrowed under the credit facility during the period.

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Date the credit facility terminates, in CCYY-MM-DD format.

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The effective interest rate at the end of the reporting period.

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The effective interest rate during the reporting period.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

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The fee, expressed as a percentage of the line of credit facility, for available but unused credit capacity under the credit facility.

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Proceeds from securitizations of receivables treated as collateralized borrowings, which are classified as financing transactions.

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Amount of cash and cash equivalents restricted as to withdrawal or usage. Cash includes, but is not limited to, currency on hand, demand deposits with banks or financial institutions, and other accounts with general characteristics of demand deposits. Cash equivalents include, but are not limited to, short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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Long-Term Debt - Additional Information (Detail) - USD ($)			1 Months Ended			6 Months Ended
	Feb. 05, 2018	Oct. 02, 2017	Feb. 28, 2018	Nov. 30, 2017	Jun. 30, 2017	Jun. 30, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Line of Credit Facility [Line Items]
Cash deposited to restricted cash reserve account					$ 10,630,000	$ 26,356,000	$ 16,787,000	$ 8,297,000
Private offering of investment grade asset-backed notes						150,000,000
Amortizing Loan [Member]
Line of Credit Facility [Line Items]
Percentage of advances on debt agreement eligible secured finance receivables			85.00%
Cash deposited to restricted cash reserve account						$ 1,300,000
Amortizing loan, amount									$ 75,700,000
Additional loan advance				$ 37,800,000
Credit agreement, Interest rate			3.25%
Debt maturity date				2024-12
Terms related to loan prepayment						The credit agreement allows the Company to prepay the loan when the outstanding balance falls below 20% of the original loan amount.
Principal balance percentage threshold for allowable prepayment of loan						20.00%
Amortizing Loan Prior to February 2018 [Member]
Line of Credit Facility [Line Items]
Percentage of advances on debt agreement eligible secured finance receivables				88.00%
Credit agreement, Interest rate				3.00%
Revolving Warehouse Credit Facility [Member]
Line of Credit Facility [Line Items]
Secured line of credit						$ 150,000,000
Percentage of advances on debt agreement eligible secured finance receivables						80.00%
Description of Variable Rate Basis						Borrowings under the facility bore interest, payable monthly, at a blended rate equal to three-month LIBOR, plus a margin of 3.50%. In October 2017 and February 2018, the margin decreased to 3.25% and 3.00%, respectively, following the satisfaction of milestones associated with the Company's conversion to a new loan origination and servicing system. The three-month LIBOR was 2.34% and 1.69% at June 30, 2018 and December 31, 2017, respectively.
Cash deposited to restricted cash reserve account						$ 400,000
Debt maturity date						2019-12
Debt revolving period end date						2018-12
Revolving Warehouse Credit Facility Prior to May 2018 [Member]
Line of Credit Facility [Line Items]
Secured line of credit						$ 125,000,000
RMIT 2018-1 securitization [Member]
Line of Credit Facility [Line Items]
Cash deposited to restricted cash reserve account						$ 1,700,000
Debt maturity date						2027-07
Terms related to loan prepayment						Prior to maturity in July 2027, the Company may redeem the notes in full, but not in part, at its option on any note payment date on or after the payment date occurring in July 2020. No payments of principal of the notes will be made during the revolving period.
Debt revolving period end date						2020-06
Private offering of investment grade asset-backed notes						$ 150,000,000
Weighted average rate						3.93%
Senior Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Secured line of credit					$ 638,000,000
Debt agreement expiration date					Jun. 20, 2020
Line of credit facility, accordion feature					$ 700,000,000
Unused line fee per annum						0.50%
Decreased unused line fee						0.375%
Percentage of advances on debt agreement eligible secured finance receivables						85.00%
Percentage of advances on debt agreement eligible unsecured finance receivables						70.00%
Current percentage of advances on eligible secured finance receivables						82.00%
Current percentage of advances on eligible unsecured finance receivables						67.00%
Credit facility, eligible borrowing capacity						$ 67,100,000
Description of Variable Rate Basis						Borrowings under the facility bear interest, payable monthly, at rates equal to LIBOR of a maturity the Company elects between one and six months, with a LIBOR floor of 1.00%, plus a 3.00% margin, increasing to 3.25% when the availability percentage is below 10%. The one-month LIBOR rate was 2.09% and 1.56% at June 30, 2018 and December 31, 2017, respectively. Alternatively, the Company may pay interest at the prime rate, plus a 2.00% margin, increasing to 2.25% when the availability percentage is below 10%. The prime rate was 5.00% and 4.50% at June 30, 2018 and December 31, 2017, respectively.
Senior Revolving Credit Facility [Member] \| Senior Revolving Credit Facility Prior to June 20, 2017 [Member]
Line of Credit Facility [Line Items]
Secured line of credit					$ 585,000,000
Debt agreement expiration date					Aug. 31, 2019
Waterfall [Member]
Line of Credit Facility [Line Items]
Cash deposited to restricted cash reserve account						$ 16,400,000	$ 8,600,000
Minimum [Member] \| Revolving Warehouse Credit Facility [Member]
Line of Credit Facility [Line Items]
Unused line fee per annum						0.35%
Minimum [Member] \| Senior Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Average outstanding amount						$ 413,000,000
Maximum [Member] \| Revolving Warehouse Credit Facility [Member]
Line of Credit Facility [Line Items]
Unused line fee per annum						0.85%
London Interbank Offered Rate (LIBOR) [Member]
Line of Credit Facility [Line Items]
Interest rate						2.09%	1.56%
London Interbank Offered Rate (LIBOR) [Member] \| Revolving Warehouse Credit Facility [Member]
Line of Credit Facility [Line Items]
Interest rate, basis spread						3.50%
Interest rate						2.34%	1.69%
London Interbank Offered Rate (LIBOR) [Member] \| Milestones Associated with Conversion to New Loan Origination and Servicing System [Member] \| Revolving Warehouse Credit Facility [Member]
Line of Credit Facility [Line Items]
Interest rate, basis spread	3.00%	3.25%
London Interbank Offered Rate (LIBOR) [Member] \| Senior Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Interest rate, basis spread						3.00%
Interest rate						2.09%	1.56%
London Interbank Offered Rate (LIBOR) [Member] \| Senior Revolving Credit Facility [Member] \| Availability Percentage Below 10% [Member]
Line of Credit Facility [Line Items]
Interest rate, basis spread						3.25%
London Interbank Offered Rate (LIBOR) [Member] \| Minimum [Member] \| Senior Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Effective interest rate on line of credit						1.00%
Prime Rate [Member] \| Senior Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Interest rate, basis spread						2.00%
Interest rate						5.00%	4.50%
Prime Rate [Member] \| Senior Revolving Credit Facility [Member] \| Availability Percentage Below 10% [Member]
Line of Credit Facility [Line Items]
Interest rate, basis spread						2.25%