Accumulated Other Comprehensive Income (Schedule of accumulated other comprehensive income (loss)) (Details) - Commodity Contract - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, before Tax	$ 0	$ 0	$ 413
Tax expense	0	0	(175)
Total reclassification for the period	0	0	238
Oil and natural gas revenue
Amounts reclassified from accumulated other comprehensive income	0	0	603
Gain (loss) on derivatives not designated as hedges and hedge ineffectiveness
Derivative Instruments, Gain (Loss) Recognized in Income	$ 0	$ 0	$ (190)