Revenues from Contracts with Customers - Schedule of Revenue Attributable to Identified Revenue Streams is Disaggregated (Parenthetical) (Detail) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Disaggregation of Revenue [Line Items]
Brokered natural gas, marketing and other	$ 98,084,000	$ 55,779,000	$ 158,063,000	$ 107,427,000
Natural gas sales
Disaggregation of Revenue [Line Items]
Brokered natural gas, marketing and other	93,400,000		149,300,000
Marketing revenues	3,900,000		7,700,000
NGLs sales
Disaggregation of Revenue [Line Items]
Brokered natural gas, marketing and other	$ 729,000		$ 1,000,000