Accumulated Other Comprehensive Income (Loss) (Schedule of accumulated other comprehensive income (loss)) (Details) (Commodity Derivatives [Member], USD $) In Thousands, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2013		Sep. 30, 2012		Sep. 30, 2013		Sep. 30, 2012
Amounts reclassified from accumulated other comprehensive income	$ (3,210)		$ 14,243		$ (202)		$ 38,088
Derivative Instruments, Gain Recognized in Income	(252)		(4,015)		116		(4,621)
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, before Tax	(3,462)		10,228		(86)		33,467
Tax expense	1,337		(3,963)		19		(13,000)
Total reclassification for the period	(2,125)		6,265		(67)		20,467
Oil and natural gas revenue [Member]
Amounts reclassified from accumulated other comprehensive income	(3,210)	[1]	14,243	[1]	(202)	[1]	38,088	[1]
Derivative Instruments, Gain Recognized in Income	0		0		0		0
Gain (loss) on derivatives not designated as hedges and hedge ineffectiveness, net [Member]
Amounts reclassified from accumulated other comprehensive income	0		0		0		0
Derivative Instruments, Gain Recognized in Income	$ (252)	[2]	$ (4,015)	[2]	$ 116	[2]	$ (4,621)	[2]

[1] Effective portion of gain (loss).

[2] Ineffective portion of gain (loss).