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Related Party Transactions - Additional Information (Detail) - WisdomTree ETF [Member] - USD ($) $ in Thousands | 3 Months Ended | 6 Months Ended |
--- | --- | --- | ---
Jun. 30, 2018 | Jun. 30, 2017 | Jun. 30, 2018 | Jun. 30, 2017 | Dec. 31, 2017
Related Party Transaction [Line Items] |  |  |  |  |
Investment | $ 2,588 |  | $ 2,588 |  | $ 1,691
Maximum [Member] |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |
Loss on investment | $ (100) | $ (100) | $ (100) | $ (100) |

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