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Income Taxes - Schedule of Changes in Balance of Gross Unrecognized Tax Benefits (Detail)
$ in Thousands
 9 Months Ended
Sep. 30, 2018
USD ($)
Income Tax Contingency [Line Items]  
Accrued in connection with the ETFS acquisition $ 38,488
Increases 227
Foreign currency translation (3,061)
Ending balance 35,654
Unrecognized Tax Benefits [Member]  
Income Tax Contingency [Line Items]  
Accrued in connection with the ETFS acquisition 31,317
Foreign currency translation (2,491)
Ending balance 28,826
Interest and Penalties [Member]  
Income Tax Contingency [Line Items]  
Accrued in connection with the ETFS acquisition 7,171
Increases 227
Foreign currency translation (570)
Ending balance $ 6,828