Acquisitions and Divestitures (Narrative) (Details)	1 Months Ended						12 Months Ended
	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($) a	Jul. 31, 2016 USD ($)	May 31, 2015 USD ($)	Apr. 30, 2015 USD ($) mi MMcf	Jan. 31, 2015 USD ($) a MMcf	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 item MMcf	Nov. 30, 2015 USD ($)
Business Acquisition [Line Items]
Repayments of long-term debt							$ 1,139,000,000	$ 1,175,000,000	$ 500,000,000
Long-term Debt [Member] | 2015 Term Loan due December 2020 [Member]
Business Acquisition [Line Items]
Repayments of long-term debt	$ 327,000,000	$ 48,000,000	$ 375,000,000				327,000,000	423,000,000
Loans payable											$ 750,000,000
WPX Property Acquisition [Member]
Business Acquisition [Line Items]
Area of land purchased | a						46,700
Cash consideration						$ 270,000,000	270,000,000
Natural gas production volume, per day | MMcf										50
Number of horizontal wells operated | item										63
Firm transportation capacity assumed, per day | MMcf						260
Firm transportation, useful life						19 years
Amortization							26,000,000	$ 17,000,000
Percentage of interest acquired									86.00%
Assets acquired and liabilities net book value							$ 270,000,000
Statoil Property Acquisition [Member]
Business Acquisition [Line Items]
Area of land purchased | a						30,000
Allocated to natural gas and oil properties						$ 357,000,000
Percentage of interest acquired						20.00%
West Virginia [Member]
Business Acquisition [Line Items]
Area of land sold | a		55,000
Proceeds from sale of oil and gas property and equipment		$ 401,000,000
Gain on sale of property		$ 0
East Texas and Arkoma Basin [Member]
Business Acquisition [Line Items]
Proceeds from sale of oil and gas property and equipment				$ 211,000,000
Reduction in capitalized costs relating to proceeds				205,000,000
Gain on sale of property				$ 0
Bradford And Lycoming Counties, Pennsylvania [Member]
Business Acquisition [Line Items]
Proceeds from sale of oil and gas property and equipment					$ 489,000,000
Net book value					206,000,000
Gain on sale of property					$ 283,000,000
Number of miles of natural gas gathering pipeline | mi					100
Net production per day of acreage sold in MMcf | MMcf					600
Bradford And Lycoming Counties, Pennsylvania [Member] | Long-term Debt [Member] | 2015 Term Loan due December 2020 [Member]
Business Acquisition [Line Items]
Loans payable											$ 500,000,000