Acquisitions (Details) a in Thousands, $ in Thousands			3 Months Ended
	Feb. 27, 2017 USD ($) a Mcfe well mi	Feb. 01, 2017 USD ($) a	Mar. 31, 2017 USD ($)	Mar. 31, 2016 USD ($)
Business Acquisition [Line Items]
Payments to acquire businesses, gross			$ 669,479	$ 0
Bankruptcy Auction
Business Acquisition [Line Items]
Area of property acquired (in acres) | a	85
Payments to acquire businesses, gross	$ 522,500
Number of acres with drilling rights (in acres) | a	44
Production of well (in MMcfe) | Mcfe	110
Number of wells acquired | well	174
Number of miles of pipeline acquired | mi	20
First Quarter 2017 Acquisitions
Business Acquisition [Line Items]
Payments to acquire businesses, gross			652,500
PP&E acquired			664,400
Current liabilities assumed			7,200
Noncurrent liabilities assumed			4,700
2016 Acquisitions
Business Acquisition [Line Items]
Payments to acquire businesses, gross			17,000
Capital expenditures incurred but not yet paid			$ 3,500
Marion, Monongalia, and Wetzel Counties
Business Acquisition [Line Items]
Area of property acquired (in acres) | a		14
Payments to acquire businesses, gross		$ 130,000