Accumulated Other Comprehensive Income (Schedule of accumulated other comprehensive income (loss)) (Details) (Commodity Contract, USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Amounts reclassified from accumulated other comprehensive income	$ 0		$ 603
Derivative Instruments, Gain (Loss) Recognized in Income	0		(190)
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, before Tax	0		413		49,237
Tax expense	0		(175)		(19,163)
Total reclassification for the period	0		238		30,074
Oil and natural gas revenue
Amounts reclassified from accumulated other comprehensive income	0	[1]	603	[1]	51,853
Gain (loss) on derivatives not designated as hedges and hedge ineffectiveness
Derivative Instruments, Gain (Loss) Recognized in Income	$ 0	[2]	$ (190)	[2]	$ (2,616)

[1] Effective portion of gain (loss).

[2] Ineffective portion of gain (loss).