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Related Party Transactions - Additional Information (Detail) - WisdomTree ETF [Member] - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Related Party Transaction [Line Items]
Investment	$ 2,720		$ 2,720		$ 1,691
Maximum [Member]
Related Party Transaction [Line Items]
Gain (Loss) on investment	$ 100	$ 100	$ 100	$ 100

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- Definition

Amount of realized and unrealized gain (loss) on investment.

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Sum of the carrying amounts as of the balance sheet date of all investments.

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