Summary of Significant Accounting Policies - Major Customers and Associated Percentage of Revenue (Detail) - Sales Revenue, Net [Member] - Customer Concentration Risk [Member]	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue, Major Customer [Line Items]
Major customers and associated percentage of revenue	25.00%	27.00%	63.00%
Antero Resources [Member]
Revenue, Major Customer [Line Items]
Major customers and associated percentage of revenue			14.00%
Concord Energy [Member]
Revenue, Major Customer [Line Items]
Major customers and associated percentage of revenue			12.00%
Emera Energy Services [Member]
Revenue, Major Customer [Line Items]
Major customers and associated percentage of revenue		17.00%
EnLink Midstream [Member]
Revenue, Major Customer [Line Items]
Major customers and associated percentage of revenue			17.00%
Marathon Petroleum [Member]
Revenue, Major Customer [Line Items]
Major customers and associated percentage of revenue	25.00%	10.00%
Sequent Energy Management [Member]
Revenue, Major Customer [Line Items]
Major customers and associated percentage of revenue			20.00%