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Related Party Transactions - Additional Information (Detail) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Dec. 31, 2018
Related Party Transaction [Line Items]          
Gain (Loss) on Investments $ (205) $ (67) $ 148 $ (58)  
WisdomTree ETF [Member]          
Related Party Transaction [Line Items]          
Investment $ 3,193   $ 3,193   $ 7,117