Derivative Instruments (Details 3) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013		Mar. 31, 2012
Before-tax effect of derivative instruments not designated as hedging instruments on the consolidated statements of operations
Unrealized gain (loss) on commodity contracts	$ (4,673)	[1]	$ 2,597	[1]
Realized gain on commodity contracts	1,869	[1]	3,175	[1]
Unrealized gain (loss) on interest rate swap	37	[2]	(23)	[2]
Realized loss on interest rate swap	(63)	[2]	(23)	[2]
Total activity for derivatives not designated as hedging instruments	$ (2,830)		$ 5,726

[1] Included in price risk management activities, net on the consolidated statements of operations. Price risk management activities totaled $(2,804) and $5,772 for the three months ended March 31, 2013 and 2012, respectively.

[2] Included in interest expense, net on the consolidated statements of operations.