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Fair Value Measurements - Additional Information (Detail) - USD ($)
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Liquid investments, original maturities 90 days    
Impairment charges $ 17,386,000   $ 1,676,000
Growth rate for deferred consideration 1.50%    
Thesys Group, Inc [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Impairment charges $ 3,829,000    
Advisor Engine [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Impairment charges 3,278,000 $ 3,278,000  
Minimum [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Gold forward-looking price range 1,294,000    
Maximum [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Gold forward-looking price range $ 2,621,000