Long-Term Debt - Additional Information (Detail) - USD ($)		12 Months Ended
Apr. 11, 2018	Dec. 31, 2018
Debt Instrument [Line Items]
Proceeds from term loan		$ 200,000,000
Interest expense		7,962,000
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Unamortized issuance costs		$ (1,195,000)
Acquisition of ETFS Business [Member] \| Term Loan Facility [Member]
Debt Instrument [Line Items]
Proceeds from term loan	$ 200,000,000
Debt instrument, interest rate		Interest on the Term Loan accrues at a rate per annum equal to LIBOR, plus up to 2.00% (commencing at LIBOR, plus 1.75%)
Debt instrument, variable rate basis		LIBOR
Acquisition of ETFS Business [Member] \| Term Loan Facility [Member] \| London Interbank Offered Rate (LIBOR) [Member]
Debt Instrument [Line Items]
Debt instrument, interest rate	1.75%
Acquisition of ETFS Business [Member] \| Term Loan Facility [Member] \| Maximum [Member] \| London Interbank Offered Rate (LIBOR) [Member]
Debt Instrument [Line Items]
Debt instrument, interest rate	2.00%
Acquisition of ETFS Business [Member] \| Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Debt instrument, interest rate		Revolver accrues at a rate per annum equal to LIBOR, plus up to 1.50% (commencing at LIBOR, plus 1.25%)
Revolving credit facility maximum borrowing capacity	$ 50,000,000
Debt instrument, facility fee rate	0.50%
Acquisition of ETFS Business [Member] \| Revolving Credit Facility [Member] \| London Interbank Offered Rate (LIBOR) [Member]
Debt Instrument [Line Items]
Debt instrument, interest rate	1.25%
Acquisition of ETFS Business [Member] \| Revolving Credit Facility [Member] \| Maximum [Member] \| London Interbank Offered Rate (LIBOR) [Member]
Debt Instrument [Line Items]
Debt instrument, interest rate	1.50%

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

No definition available.

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- Definition

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.

+ References

No definition available.

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- Definition

Description of the interest rate as being fixed or variable, and, if variable, identification of the index or rate on which the interest rate is based and the number of points or percentage added to that index or rate to set the rate, and other pertinent information, such as frequency of rate resets.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
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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.

+ References

No definition available.

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- Definition

Amount of the cost of borrowed funds accounted for as interest expense.

+ References

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-Publisher FASB
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-Topic 835
-SubTopic 20
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- Definition

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.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
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-Paragraph 1
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- Definition

The cash inflow from a debt initially having maturity due after one year or beyond the operating cycle, if longer.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
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- Definition

Debt Instrument Fee Percentage

+ References

No definition available.

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- Definition

The remaining balance of debt issuance costs associated with the revolving credit facility that are being amortized against income over its life.

+ References

No definition available.

+ Details

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