Condensed Combined Statement of Income for LLCs/LPs Accounted Under Equity Method (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Equity Method Investments And Joint Ventures [Abstract]
Revenues	$ 2,531	$ 2,451	$ 4,991	$ 4,913
Operating expenses	994	977	2,006	1,935
Depreciation and amortization	485	438	883	895
Interest, net	322	328	644	656
Net income	730	708	1,458	1,427
Our share of net income	$ 454	$ 425	$ 884	$ 854