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Income Taxes - Schedule of Changes in Balance of Gross Unrecognized Tax Benefits (Detail)
$ in Thousands
 12 Months Ended
Dec. 31, 2018
USD ($)
Income Tax Contingency [Line Items]  
Accrued in connection with the ETFS Acquisition $ 38,488
Increases 346
Foreign currency translation (3,958)
Ending balance 34,876
Unrecognized Tax Bene fits [Member]  
Income Tax Contingency [Line Items]  
Accrued in connection with the ETFS Acquisition 31,317
Foreign currency translation (3,216)
Ending balance 28,101
Interest And Penalties [Member]  
Income Tax Contingency [Line Items]  
Accrued in connection with the ETFS Acquisition 7,171
Increases 346
Foreign currency translation (742)
Ending balance $ 6,775