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Revenues from Contracts with Customers - Summary of Revenues from Contracts with Customers (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Revenues Categorized as Revenues from Contracts with Customers and Other Sources of Revenues [Line Items]      
Total operating revenues $ 274,116 $ 228,295 $ 218,920
Advisory Fees [Member]      
Revenues Categorized as Revenues from Contracts with Customers and Other Sources of Revenues [Line Items]      
Total operating revenues 271,104 226,692 218,217
Other Income [Member]      
Revenues Categorized as Revenues from Contracts with Customers and Other Sources of Revenues [Line Items]      
Total operating revenues $ 3,012 $ 1,603 $ 703