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				6 Months Ended
	Jun. 30, 2013		Jun. 30, 2012		Jun. 30, 2013		Jun. 30, 2012
Amounts reclassified from accumulated other comprehensive income	$ (823)		$ 15,670		$ 3,008		$ 23,846
Derivative Instruments, Gain Recognized in Income	1,717		1,387		368		(606)
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, before Tax	894		17,057		3,376		23,240
Tax expense	(350)		(6,642)		(1,318)		(9,038)
Total reclassification for the period	544		10,415		2,058		14,202
Oil and natural gas revenue [Member]
Amounts reclassified from accumulated other comprehensive income	(823)	[1]	15,670	[1]	3,008	[1]	23,846	[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	$ 1,717	[2]	$ 1,387	[2]	$ 368	[2]	$ (606)	[2]

[1] Effective portion of gain (loss).

[2] Ineffective portion of gain (loss).