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Revenues from Contracts with Customers - Summary of Revenues from Contracts with Customers (Detail) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Revenues Categorized as Revenues from Contracts with Customers and Other Sources of Revenues [Line Items]        
Total operating revenues $ 72,570 $ 57,714 $ 206,249 $ 167,322
Advisory Fees [Member]        
Revenues Categorized as Revenues from Contracts with Customers and Other Sources of Revenues [Line Items]        
Total operating revenues 71,679 57,293 203,913 166,177
Other Income [Member]        
Revenues Categorized as Revenues from Contracts with Customers and Other Sources of Revenues [Line Items]        
Total operating revenues $ 891 $ 421 $ 2,336 $ 1,145