Acquisitions And Divestitures (Narrative) (Details) $ in Millions		1 Months Ended					3 Months Ended								5 Months Ended	12 Months Ended
	Dec. 19, 2014 USD ($)	May. 31, 2015 USD ($)	Apr. 30, 2015 USD ($) mi MMcf	Jan. 31, 2015 USD ($) a MMcf	Dec. 31, 2014 USD ($) a MMcf	Apr. 30, 2013 USD ($)	Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($) a MMcf	Sep. 30, 2014 USD ($)	Jun. 30, 2014 USD ($)	Mar. 31, 2014 USD ($)	Jul. 31, 2014 USD ($) a	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($) a item MMcf	Dec. 31, 2013 USD ($)
Business Acquisition [Line Items]
Debt issuance costs				$ 47.0
General and administrative expenses																$ 246.0	$ 221.0	$ 191.0
Interest expense																56.0	59.0	42.0
Other current assets					$ 158.0		$ 45.0				$ 158.0					45.0	158.0
Revenues							687.0	$ 749.0	$ 764.0	$ 933.0	962.0	$ 928.0	$ 1,035.0	$ 1,113.0		3,133.0	4,038.0	3,371.0
Operating income (loss)							$ (2,561.0)	$ (2,842.0)	$ (1,284.0)	$ 165.0	285.0	$ 286.0	$ 367.0	$ 435.0		(6,522.0)	$ 1,373.0	$ 1,204.0
Bridge Loan [Member]
Business Acquisition [Line Items]
Maturity date, month and year																	2015-12
Bridge loan	$ 4,500.0				4,500.0						4,500.0						$ 4,500.0
Debt instrument, term	364 days																364 days
Repayment of debt				4,500.0
Debt issuance costs				$ 47.0
Unsecured Debt [Member] | 1.545% Term Loan due December 2016 [Member]
Business Acquisition [Line Items]
Loans payable					$ 500.0						$ 500.0						$ 500.0
Maturity date, month and year																	2016-12
Debt instrument, term																	2 years
WPX Property Acquisition [Member]
Business Acquisition [Line Items]
Area of land purchased | a				46,700
Cash consideration				$ 270.0
Net production per day of acreage sold in MMcf | MMcf					50						50						50
Number of horizontal wells operated | item																	63
Firm transportation capacity assumed, per day | MMcf				260
Firm transportation, useful life				19 years
Amortization																$ 8.2
Percentage of interest acquired							86.00%									86.00%
Statoil Property Acquisition [Member]
Business Acquisition [Line Items]
Area of land purchased | a				30,000
Allocated to natural gas and oil properties				$ 357.0
Percentage of interest acquired				20.00%
Cost to acquire property				$ 357.0
Chesapeake Property Acquisition [Member]
Business Acquisition [Line Items]
Area of land purchased | a					413,000						413,000						413,000
Cash consideration					$ 4,949.0
General and administrative expenses																	$ 1.0
Interest expense																	5.0
Revenues																	10.0
Operating income (loss)																	2.0
Chesapeake Property Acquisition [Member] | Bridge Loan [Member]
Business Acquisition [Line Items]
Other current assets					47.0						$ 47.0						47.0
Chesapeake Property Acquisition [Member] | Unsecured Debt [Member] | 1.545% Term Loan due December 2016 [Member]
Business Acquisition [Line Items]
Unamortized fees					$ 1.0						$ 1.0						$ 1.0
East Texas and Arkoma Basin [Member]
Business Acquisition [Line Items]
Proceeds from sale of oil and gas property and equipment		$ 211.0
Reduction in capitalized costs relating to proceeds		$ (205.0)
Bradford And Lycoming Counties, Pennsylvania [Member]
Business Acquisition [Line Items]
Proceeds from sale of oil and gas property and equipment			$ 489.0
Net book value			206.0
Gain on sale of property			$ 283.0
Number of miles of natural gas gathering pipeline | mi			100
Pipeline capacity per day | MMcf			600
Niobrara Formation In Colorado [Member]
Business Acquisition [Line Items]
Area of land purchased | a															380,000
Cost to acquire natural gas properties through definitive purchase agreement															$ 215.0
Marcellus Shale In Pennsylvania [Member]
Business Acquisition [Line Items]
Cost to acquire natural gas properties through definitive purchase agreement						$ 82.0