Derivative Instruments/Hedging (Details 2) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Unrealized gains (losses) on fair value change:
Total unrealized gains (losses) on fair value change			$ 10,292	$ 5,990
Derivatives not designated as hedging instruments
Settlements gains (losses):
Total settlements gains (losses)	1,039		1,765
Unrealized gains (losses) on fair value change:
Total unrealized gains (losses) on fair value change	12,621	10,078	10,292	5,990
Gain (loss) on derivative instruments, net	13,660	10,078	12,057	5,990
Natural gas contracts | Derivatives not designated as hedging instruments
Unrealized gains (losses) on fair value change:
Total unrealized gains (losses) on fair value change	9,199		9,199
Oil contracts | Derivatives not designated as hedging instruments
Settlements gains (losses):
Total settlements gains (losses)	1,039		1,765
Unrealized gains (losses) on fair value change:
Total unrealized gains (losses) on fair value change	$ 3,422	$ 10,078	$ 1,093	$ 5,990