Revenues from Contracts with Customers - Summary of Revenues from Contracts with Customers (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Revenues Categorized as Revenues from Contracts with Customers and Other Sources of Revenues [Line Items]
Total operating revenues	$ 64,640	$ 67,718	$ 186,640	$ 199,496
Advisory Fees [Member]
Revenues Categorized as Revenues from Contracts with Customers and Other Sources of Revenues [Line Items]
Total operating revenues	63,919	67,006	184,077	197,473
Other Income [Member]
Revenues Categorized as Revenues from Contracts with Customers and Other Sources of Revenues [Line Items]
Total operating revenues	$ 721	$ 712	$ 2,563	$ 2,023