Revenues from Contracts with Customers - Summary of Revenues from Contracts with Customers (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Revenues Categorized as Revenues from Contracts with Customers and Other Sources of Revenues [Line Items]
Total operating revenues	$ 65,485	$ 58,904
Advisory Fees [Member]
Revenues Categorized as Revenues from Contracts with Customers and Other Sources of Revenues [Line Items]
Total operating revenues	64,840	58,456
Other Income [Member]
Revenues Categorized as Revenues from Contracts with Customers and Other Sources of Revenues [Line Items]
Total operating revenues	$ 645	$ 448