Natural Gas And Oil Properties (Narrative) (Details) $ in Millions	6 Months Ended
	Jun. 30, 2016 USD ($) $ / MMBTU $ / bbl	Jun. 30, 2015 USD ($) $ / MMBTU $ / bbl	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)
Natural Gas and Oil Properties [Line Items]
Natural gas, oil and NGL reserves discount	10.00%
Period of time needed to calculate ceiling value of reserves	12 months	12 months
Cash flow hedges impact on ceiling value, net of tax | $		$ 60
Impairment of natural gas and oil properties, net of tax | $	$ 297	$ 944	$ 641	$ 1,586	$ 1,746
Natural Gas [Member] | Henry Hub [Member]
Natural Gas and Oil Properties [Line Items]
Full cost ceiling test, price | $ / MMBTU	2.24	3.39
Oil [Member] | West Texas Intermediate [Member]
Natural Gas and Oil Properties [Line Items]
Full cost ceiling test, price | $ / bbl	39.63	68.17
NGL [Member]
Natural Gas and Oil Properties [Line Items]
Full cost ceiling test, price | $ / bbl	5.87	12.53