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Related Party Transactions - Additional Information (Detail) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Sep. 30, 2019 | Sep. 30, 2018 | Sep. 30, 2019 | Sep. 30, 2018 | Dec. 31, 2018
Related Party Transaction [Line Items] |  |  |  |  |
Gains on Investments | $ 64 | $ 76 | $ 212 | $ 17 |
WisdomTree ETF [Member] |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |
Investment | $ 2,927 |  | $ 2,927 |  | $ 7,117

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