Acquisitions (Details) $ / shares in Units, Bcfe in Thousands, $ in Thousands						3 Months Ended	12 Months Ended
	Jun. 30, 2017 a	Feb. 27, 2017 a Bcfe well mi	Feb. 01, 2017 a	Dec. 16, 2016 a well	Jul. 08, 2016 a well	Dec. 31, 2016 USD ($) a well	Dec. 31, 2017 USD ($) a	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Oct. 27, 2016 $ / shares
Business Acquisition [Line Items]
Number of acres acquired (in acres) | a							304,000
Payments to acquire businesses, net							$ 1,560,272	$ 0	$ 0
Adjustment to PPE							4,400	(27,700)	$ (114,800)
Marion, Monongalia, and Wetzel Counties
Business Acquisition [Line Items]
Number of acres acquired (in acres) | a			14,000
Statoil Acquisition
Business Acquisition [Line Items]
Number of acres acquired (in acres) | a					62,500
Number of wells acquired | well					31
Number of producing wells acquired | well					24
Republic and Trans Energy Transactions
Business Acquisition [Line Items]
Number of acres acquired (in acres) | a						42,600
Number of wells acquired | well						42
Number of producing wells acquired | well						32
Trans Energy Merger
Business Acquisition [Line Items]
Share price (in dollars per share) | $ / shares										$ 3.58
Pennsylvania Acquisition
Business Acquisition [Line Items]
Number of acres acquired (in acres) | a				17,000
Number of producing wells acquired | well				2
Bankruptcy Auction
Business Acquisition [Line Items]
Number of acres acquired (in acres) | a		85,000
Number of wells acquired | well		174
Number of producing wells acquired | well		120
Number of acres with drilling rights | a		44,000
Production of well (in MMcfe) | Bcfe		110
Number of miles of pipeline acquired | mi		20
Allegheny, Washington, and Westmore Counties
Business Acquisition [Line Items]
Number of acres acquired (in acres) | a	11,000
2017 Acquisitions
Business Acquisition [Line Items]
Payments to acquire businesses, net							740,100
Net property, plant and equipment							750,100
Current liabilities							5,300
Noncurrent liabilities assumed							4,700
2016 Acquisitions
Business Acquisition [Line Items]
Payments to acquire businesses, net								1,130,100
Net property, plant and equipment						$ 1,203,400		1,203,400
Current liabilities						14,400		14,400
Noncurrent liabilities assumed						11,100		11,100
Payments to acquire land								78,900
Adjustment to PPE							$ 14,300
Other noncurrent assets acquired						1,200		1,200
Notes payable assumed						5,100		5,100
Deferred income taxes						49,000		49,000
2016 Acquisitions | Proved Property
Business Acquisition [Line Items]
Net property, plant and equipment						256,200		256,200
2016 Acquisitions | Unproved Property
Business Acquisition [Line Items]
Net property, plant and equipment						$ 947,200		$ 947,200