Acquisitions (Details) a in Thousands, Mcfe in Thousands, $ in Thousands				9 Months Ended
	Jun. 30, 2017 USD ($) a	Feb. 27, 2017 USD ($) a Mcfe well mi	Feb. 01, 2017 USD ($) a	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)
Business Acquisition [Line Items]
Payments to acquire businesses, gross				$ 818,957	$ 412,348
Bankruptcy Auction
Business Acquisition [Line Items]
Area of property acquired (in acres) | a		85
Payments to acquire businesses, gross		$ 523,500
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
Allegheny, Washington, and Westmore Counties
Business Acquisition [Line Items]
Area of property acquired (in acres) | a	11
Payments to acquire businesses, gross	$ 83,700
2017 Acquisitions
Business Acquisition [Line Items]
Payments to acquire businesses, gross				740,100
PP&E acquired	750,100
Current liabilities assumed	5,300
Noncurrent liabilities assumed	$ 4,700
2016 Acquisitions
Business Acquisition [Line Items]
Payments to acquire businesses, gross				78,900
Adjustment to PP&E				$ 2,500
Marion, Monongalia, and Wetzel Counties
Business Acquisition [Line Items]
Area of property acquired (in acres) | a			14
Payments to acquire businesses, gross			$ 132,900