Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

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Goodwill and Intangible Assets - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Dec. 31, 2019	Apr. 11, 2018
Indefinite-lived Intangible Assets [Line Items]
Goodwill	$ 85,856	$ 85,856
Intangible asset related to its customary advisory agreement	601,247	603,294
Acquisition of ETFS Business [Member]
Indefinite-lived Intangible Assets [Line Items]
Goodwill			$ 84,057
Intangible asset related to its customary advisory agreement	$ 601,247	$ 601,247
Weighted average cost of capital percentage	11.60%
Acquisition of ETFS Business [Member] \| Minimum [Member]
Indefinite-lived Intangible Assets [Line Items]
Revenue growth multiple	3.00%
Acquisition of ETFS Business [Member] \| Maximum [Member]
Indefinite-lived Intangible Assets [Line Items]
Revenue growth multiple	4.00%