Accumulated Other Comprehensive Income (Schedule of accumulated other comprehensive income (loss)) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Amounts reclassified from accumulated other comprehensive income	$ 603		$ 51,853		$ 2,965
Commodity Derivatives [Member]
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, before Tax	413		49,237		5,714
Tax expense	(175)		(19,163)		(2,207)
Total reclassification for the period	238		30,074		3,507
Oil and natural gas revenue [Member] | Commodity Derivatives [Member]
Amounts reclassified from accumulated other comprehensive income	603	[1]	51,853	[1]	2,965	[1]
Gain (loss) on derivatives not designated as hedges and hedge ineffectiveness [Member] | Commodity Derivatives [Member]
Derivative Instruments, Gain Recognized in Income	$ (190)	[2]	$ (2,616)	[2]	$ 2,749	[2]

[1] Effective portion of gain (loss).

[2] Ineffective portion of gain (loss).