Natural Gas And Oil Properties (Narrative) (Details) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2016 USD ($) $ / bbl $ / MMBTU	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Sep. 30, 2017 $ / bbl $ / MMBTU	Sep. 30, 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
Impairment of natural gas and oil properties | $	$ 817	$ 470	$ 1,034		$ 2,321
Natural Gas [Member] | Henry Hub [Member]
Natural Gas and Oil Properties [Line Items]
Full cost ceiling test, price | $ / MMBTU	2.28			3.00
Oil [Member] | West Texas Intermediate [Member]
Natural Gas and Oil Properties [Line Items]
Full cost ceiling test, price	38.17			46.27
NGL [Member]
Natural Gas and Oil Properties [Line Items]
Full cost ceiling test, price	6.46			12.47