Commitments and Contingencies (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended	24 Months Ended
	May 31, 2017	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2015	Dec. 31, 2016	Dec. 31, 2017
Other Commitments [Line Items]
Firm transportation liability		$ 0	$ 0	$ (41,000)	$ 0
October - December 2017		221,000		221,000
2018		789,000		789,000
2019		501,000		501,000
2020		349,000		349,000
2021		281,000		281,000
2022 and Thereafter		1,869,000		1,869,000
Other Commitment		4,010,000		4,010,000
Transportation Agreement Commitments
Other Commitments [Line Items]
Other Commitment		1,600,000		1,600,000
Drilling, Equipment, and Purchase Obligations
Other Commitments [Line Items]
October - December 2017		136,000		136,000
2018		425,000		425,000
2019		148,000		148,000
2020		26,000		26,000
2021		7,000		7,000
2022 and Thereafter		36,000		36,000
Other Commitment		778,000		778,000
Transportation and Gathering Obligations
Other Commitments [Line Items]
October - December 2017		53,000		53,000
2018		247,000		247,000
2019		276,000		276,000
2020		249,000		249,000
2021		213,000		213,000
2022 and Thereafter		1,499,000		1,499,000
Other Commitment		2,537,000		2,537,000
Operating Lease Obligations
Other Commitments [Line Items]
October - December 2017		12,000		12,000
2018		43,000		43,000
2019		32,000		32,000
2020		32,000		32,000
2021		32,000		32,000
2022 and Thereafter		189,000		189,000
Other Commitment		340,000		340,000
Capital Lease and Other Obligations
Other Commitments [Line Items]
October - December 2017		20,000		20,000
2018		74,000		74,000
2019		45,000		45,000
2020		42,000		42,000
2021		29,000		29,000
2022 and Thereafter		145,000		145,000
Other Commitment		$ 355,000		355,000
Consent Decree
Other Commitments [Line Items]
Civil penalty						$ 4,950	$ 54,700
Mitigation projects				4,500
Supplemental environmental projects				$ 4,000
Maximum | Marcellus Shale | Transportation Agreement Commitments
Other Commitments [Line Items]
Term				15 years
Scenario, Forecast | Transportation Agreement Commitments
Other Commitments [Line Items]
Other Commitment								$ 627,000
Civil Penalty
Other Commitments [Line Items]
Settlement amount	$ 24,710
Payment to approved SEP
Other Commitments [Line Items]
Settlement amount	$ 98,840