Goodwill and Intangible Assets - Summary of Indefinite-lived Intangible Assets (Detail) $ in Thousands	9 Months Ended
	Sep. 30, 2018 USD ($)
Indefinite-lived Intangible Assets [Line Items]
Beginning balance	$ 12,085
Foreign currency translation	(58)
Other increases/(decreases)	601,247
Ending balance	613,274
Acquisition of ETFS Business [Member]
Indefinite-lived Intangible Assets [Line Items]
Other increases/(decreases)	601,247
Ending balance	601,247
GreenHaven Commodity Services, LLC [Member]
Indefinite-lived Intangible Assets [Line Items]
Beginning balance	9,953
Ending balance	9,953
Questrade ETFs [Member]
Indefinite-lived Intangible Assets [Line Items]
Beginning balance	2,132
Foreign currency translation	(58)
Ending balance	$ 2,074