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Organizational and Description of Business - Additional Information (Detail) - Acquisition of ETFS Business [Member] - USD ($)
3 Months Ended
Nov. 13, 2017
Mar. 31, 2018
Apr. 11, 2018
Organization Consolidation And Presentation Of Financial Statements [Line Items]      
Acquisition completion date   Apr. 11, 2018  
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 common shares that the preferred stock is convertible into 14,750,000    
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    
Revolving Credit Facility [Member]      
Organization Consolidation And Presentation Of Financial Statements [Line Items]      
Revolving credit facility maximum borrowing capacity     $ 50,000,000
Term Loan Facility [Member]      
Organization Consolidation And Presentation Of Financial Statements [Line Items]      
Borrowing under term loan facility     $ 200,000,000
Common Stock [Member]      
Organization Consolidation And Presentation Of Financial Statements [Line Items]      
Business acquisition, number of shares to be issued 15,250,000