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Goodwill and Intangible Assets - Additional Information (Detail) - USD ($)
$ in Thousands
Jun. 30, 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