Sales of Assets (Details) a in Thousands				12 Months Ended
	Dec. 16, 2016 USD ($) a	Mar. 31, 2012	Mar. 26, 2012 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Proceeds from asset sales				$ 171,830,000	$ 40,000,000
Gain on sale of assets				$ 97,635,000		$ 40,000,000
Pennsylvania Leasehold Acreage
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Proceeds from asset sales	$ 169,800,000
Gain on sale of assets	$ 99,000,000
Area of land sold | a	17
Appalachian Gathering Assets
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Proceeds from asset sales			$ 375,000,000		40,000,000
Gain on sale of assets					$ 40,000,000
Period for which entity is committed to deliver minimum annual volumes into gathering systems, with certain carryback and carryforward adjustments for overages or deficiencies		7 years
Period of exclusive rights to gather the gas within AOD		20 years