Natural Gas And Oil Properties (Narrative) (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Jun. 30, 2017 $ / bbl $ / MMBTU	Jun. 30, 2016 USD ($) $ / bbl $ / MMBTU
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 | $	$ 470	$ 1,034		$ 1,504
Natural Gas [Member] | Henry Hub [Member]
Natural Gas and Oil Properties [Line Items]
Full cost ceiling test, price | $ / MMBTU			3.01	2.24
Oil [Member] | West Texas Intermediate [Member]
Natural Gas and Oil Properties [Line Items]
Full cost ceiling test, price			45.42	39.63
NGL [Member]
Natural Gas and Oil Properties [Line Items]
Full cost ceiling test, price			10.90	5.87