Revenues from Contracts with Customers - Summary of Revenues from Contracts with Customers (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Revenues Categorized as Revenues from Contracts with Customers and Other Sources of Revenues [Line Items]
Total operating revenues	$ 72,830	$ 63,874
Advisory Fees [Member]
Revenues Categorized as Revenues from Contracts with Customers and Other Sources of Revenues [Line Items]
Total operating revenues	71,616	62,950
Other Income [Member]
Revenues Categorized as Revenues from Contracts with Customers and Other Sources of Revenues [Line Items]
Total operating revenues	$ 1,214	$ 924