Natural Gas And Oil Properties (Narrative) (Details) $ in Millions	9 Months Ended
	Sep. 30, 2016 USD ($) $ / bbl $ / MMBTU	Sep. 30, 2015 USD ($) $ / bbl $ / MMBTU	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)
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 | $		$ 40
Impairment of natural gas and oil properties, net of tax | $	$ 506	$ 1,746	$ 297	$ 641
Natural Gas [Member] | Henry Hub [Member]
Natural Gas and Oil Properties [Line Items]
Full cost ceiling test, price | $ / MMBTU	2.28	3.06
Oil [Member] | West Texas Intermediate [Member]
Natural Gas and Oil Properties [Line Items]
Full cost ceiling test, price | $ / bbl	38.17	55.73
NGL [Member]
Natural Gas and Oil Properties [Line Items]
Full cost ceiling test, price | $ / bbl	6.46	8.62