Income Taxes - Additional Information (Detail) € in Thousands, $ in Thousands	3 Months Ended			9 Months Ended
	Sep. 30, 2018 USD ($)	Sep. 30, 2018 EUR (€)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Apr. 11, 2018 USD ($)	Apr. 11, 2018 EUR (€)
Schedule Of Income Tax [Line Items]
Estimated effective rate	19.90%	19.90%	48.50%	24.30%	48.70%
Income tax expense	$ 5,481		$ 7,520	$ 15,439	$ 25,582
Federal statutory rate	21.00%	21.00%	35.00%	21.00%	35.00%
Pre-tax federal net operating losses	$ 3,147			$ 3,147
Pre-tax federal net operating losses expiration date				2024
Tax effected NOLs - Foreign				$ 5,747
Accrued for unrecognized tax benefits and interest/penalties related to a tax position claimed or expected to be claimed on a tax return	35,654			35,654
Unrecognized tax benefits that would impact effective tax rate	28,826			28,826
Acquisition of ETFS Business [Member]
Schedule Of Income Tax [Line Items]
Accrued uncertain tax position liability						$ 25,631	€ 18,102
Interest and penalties						$ 4,835	€ 3,415
Adjustment to unrecognized tax benefits accrued uncertain tax position liability	5,686	€ 4,016
Adjustment to unrecognized tax benefits accrued interest and penalties	2,336	€ 1,650
Other Noncurrent Liabilities [Member]
Schedule Of Income Tax [Line Items]
Accrued for unrecognized tax benefits and interest/penalties related to a tax position claimed or expected to be claimed on a tax return	$ 35,654			$ 35,654