Business Combination - Additional Information (Detail) - USD ($)		3 Months Ended		6 Months Ended	9 Months Ended
	Apr. 11, 2018	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2018	Sep. 30, 2017
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Acquisition related costs		$ 456,000			$ 10,446,000
Gain (loss) on revaluation of deferred consideration		7,732,000		$ 17,630,000	17,630,000
Acquisition of ETFS Business [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Business acquisition, cash paid	$ 253,000,000
Cash consideration paid from proceeds arising from maturities of securities owned, at fair value	$ 53,000,000
Business acquisition, number of shares to be issued	30,000,000
Business acquisition, number of common shares that the preferred stock is convertible into	14,750,000
Acquisition related costs		$ 456,000			10,446,000
Gain (loss) on revaluation of deferred consideration			$ (4,869,000)		$ 4,670,000	$ (15,088,000)
Acquisition of ETFS Business [Member] | Term Loan Facility [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Borrowing under term loan facility	$ 200,000,000
Acquisition of ETFS Business [Member] | Revolving Credit Facility [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Revolving credit facility maximum borrowing capacity	$ 50,000,000
Acquisition of ETFS Business [Member] | Series A Non-Voting Convertible Preferred Stock [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Business acquisition, number of shares to be issued	14,750
Acquisition of ETFS Business [Member] | Common Stock [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Business acquisition, number of shares to be issued	15,250,000