Supplemental Oil And Gas Disclosures (Schedule Of The Oil And Natural Gas Property Costs Not Being Amortized) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Unproved properties acquired and wells in progress	$ 296,764	$ 314,867	$ 337,099
2017
Unproved properties acquired and wells in progress	50,447
2016
Unproved properties acquired and wells in progress	22,092
2015
Unproved properties acquired and wells in progress	40,254
2014 and prior
Unproved properties acquired and wells in progress	$ 183,971