Revenues from Contracts with Customers - Summary of Revenues from Contracts with Customers (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenues Categorized as Revenues from Contracts with Customers and Other Sources of Revenues [Line Items]
Total operating revenues	$ 68,907	$ 67,718	$ 66,293	$ 65,485	$ 67,867	$ 72,570	$ 74,775	$ 58,904	$ 268,403	$ 274,116	$ 228,295
Advisory Fees [Member]
Revenues Categorized as Revenues from Contracts with Customers and Other Sources of Revenues [Line Items]
Total operating revenues									265,652	271,104	226,692
Other Income [Member]
Revenues Categorized as Revenues from Contracts with Customers and Other Sources of Revenues [Line Items]
Total operating revenues									$ 2,751	$ 3,012	$ 1,603