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Revenues from Contracts with Customers - Summary of Revenues from Contracts with Customers (Detail) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Revenues Categorized as Revenues from Contracts with Customers and Other Sources of Revenues [Line Items]        
Total operating revenues $ 74,775 $ 56,242 $ 133,679 $ 109,609
Advisory Fees [Member]        
Revenues Categorized as Revenues from Contracts with Customers and Other Sources of Revenues [Line Items]        
Total operating revenues 73,778 55,856 132,234 108,884
Other Income [Member]        
Revenues Categorized as Revenues from Contracts with Customers and Other Sources of Revenues [Line Items]        
Total operating revenues $ 997 $ 386 $ 1,445 $ 725