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Business Combination - Summary of Operating Results of ETFS since the Acquisition Date (Detail) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Business Combinations [Abstract]        
Revenues       $ 18,218
Income before taxes       18,102
Gain on revaluation of deferred consideration $ (4,037) $ 9,898 $ 367 $ 9,898