Natural Gas And Oil Properties (Narrative) (Details) $ in Millions	3 Months Ended
	Mar. 31, 2017 $ / bbl $ / MMBTU	Mar. 31, 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, net of tax | $		$ 641
Natural Gas [Member] | Henry Hub [Member]
Natural Gas and Oil Properties [Line Items]
Full cost ceiling test, price | $ / MMBTU	2.73	2.40
Oil [Member] | West Texas Intermediate [Member]
Natural Gas and Oil Properties [Line Items]
Full cost ceiling test, price	44.10	42.77
NGL [Member]
Natural Gas and Oil Properties [Line Items]
Full cost ceiling test, price	10.17	5.76