Revenues from Contracts with Customers - Schedule of Revenue Attributable to Identified Revenue Streams is Disaggregated (Parenthetical) (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Disaggregation of Revenue [Line Items]
Brokered natural gas, marketing and other	$ 59,979,000	$ 51,648,000
Natural gas sales
Disaggregation of Revenue [Line Items]
Brokered natural gas, marketing and other	55,900,000
Marketing revenues	3,800,000
NGLs sales
Disaggregation of Revenue [Line Items]
Brokered natural gas, marketing and other	$ 304,000