Summary Of Significant Accounting Policies (Narrative) (Details) - USD ($)	3 Months Ended		12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Jan. 31, 2017
Summary Of Significant Accounting Policies [Line Items]
Other receivables	$ 69,000,000		$ 69,000,000	$ 30,000,000
Derivative collateral held	0		0	75,000,000
Cash collateral posted	17,000,000		17,000,000
Gains on asset sales	540,000,000	$ 1,400,000,000	1,893,000,000		$ 1,072,000,000
Debt issuance costs	$ 44,000,000		44,000,000	57,000,000
Debt retirement payments			265,000,000		51,000,000
Accounting Standards Update 2015-03 [Member]
Summary Of Significant Accounting Policies [Line Items]
Debt issuance costs				81,000,000
Accounting Standards Update 2016-15 [Member]
Summary Of Significant Accounting Policies [Line Items]
Debt retirement payments			$ 265,000,000		$ 40,000,000
Minimum [Member]
Summary Of Significant Accounting Policies [Line Items]
Depletion calculation holding period			3 years
Other property and equipment, useful life			3 years
Finite lived intangible asset useful life			10 years
Maximum [Member]
Summary Of Significant Accounting Policies [Line Items]
Depletion calculation holding period			4 years
Other property and equipment, useful life			60 years
Finite lived intangible asset useful life			20 years
Subsequent Event [Member]
Summary Of Significant Accounting Policies [Line Items]
Cash collateral posted						$ 0
Accrued Derivative Settlements [Member]
Summary Of Significant Accounting Policies [Line Items]
Other receivables				$ 236,000,000
Major Customer Accounting For More Than 10 Percent Of Operating Revenues [Member]
Summary Of Significant Accounting Policies [Line Items]
Concentration risk percentage			0.00%	0.00%	0.00%