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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, 2021
 [1]
Sep. 30, 2021
 [1]
Jun. 30, 2021
 [1]
Mar. 31, 2021
 [1]
Dec. 31, 2020
 [1]
Sep. 30, 2020
 [1]
Jun. 30, 2020
 [1]
Mar. 31, 2020
 [1]
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Revenues Categorized as Revenues from Contracts with Customers and Other Sources of Revenues [Line Items]                      
Total operating revenues $ 79,175 $ 78,112 $ 75,775 $ 71,256 $ 65,651 $ 63,749 $ 57,312 $ 63,200 $ 304,318 $ 249,912 $ 266,528
Advisory Fees [Member]                      
Revenues Categorized as Revenues from Contracts with Customers and Other Sources of Revenues [Line Items]                      
Total operating revenues                 298,052 246,395 263,777
Other Income [Member]                      
Revenues Categorized as Revenues from Contracts with Customers and Other Sources of Revenues [Line Items]                      
Total operating revenues                 $ 6,266 $ 3,517 $ 2,751
[1] Advisory fees previously reported have been revised due to an immaterial error correction. These revisions had no effect on previously reported net income. See Note 2 for additional information.