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Revenues from Contracts with Customers - Summary of Revenues from Contracts with Customers (Detail) - USD ($)
$ in Thousands
 3 Months Ended 12 Months Ended
Dec. 31, 2020
Sep. 30, 2020
Jun. 30, 2020
Mar. 31, 2020
Dec. 31, 2019
Sep. 30, 2019
Jun. 30, 2019
Mar. 31, 2019
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Revenues Categorized as Revenues from Contracts with Customers and Other Sources of Revenues [Line Items]                      
Total operating revenues $ 67,059 $ 64,640 $ 58,126 $ 63,874 $ 68,907 $ 67,718 $ 66,293 $ 65,485 $ 253,699 $ 268,403 $ 274,116
Advisory Fees [Member]                      
Revenues Categorized as Revenues from Contracts with Customers and Other Sources of Revenues [Line Items]                      
Total operating revenues                 250,182 265,652 271,104
Other Income [Member]                      
Revenues Categorized as Revenues from Contracts with Customers and Other Sources of Revenues [Line Items]                      
Total operating revenues                 $ 3,517 $ 2,751 $ 3,012