Condensed Combined Statement of Income for LLCs/LPs Accounted Under Equity Method (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Schedule Of Equity Method Investments [Line Items]
Revenues	$ 19,692	$ 19,866	$ 58,180	$ 58,304
Net income			14,447	13,126
Our share of net income	517	453	1,371	1,337
Equity Method Investment, Nonconsolidated Investee or Group of Investees
Schedule Of Equity Method Investments [Line Items]
Revenues	2,624	2,441	7,637	7,432
Operating expenses	1,047	927	3,094	2,933
Depreciation and amortization	444	434	1,333	1,317
Interest, net	315	325	949	969
Net income	818	755	2,261	2,213
Our share of net income	$ 517	$ 453	$ 1,371	$ 1,337