Derivative Instruments (Tables)

12 Months Ended

Dec. 31, 2012

Derivative Instruments and Hedging Activities Disclosure [Abstract]

Schedule of commodity derivatives

	2013	2014	2015
Oil Sales (Bbl/D):	18,800	7,500	3,000
Natural Gas Purchases (MMBtu/D):	10,000	—	—

Schedule of derivative assets and liabilities

	December 31, 2012
	Derivative Assets			Derivative Liabilities
(in millions)	Balance Sheet Classification	Fair Value		Balance Sheet Classification	Fair Value
Current:
Commodity	Derivative assets	$	16.4	Derivative liabilities	$	2.8
Long-term:
Commodity	Derivative assets	10.9		Derivative liabilities	1.2
Total derivatives		$	27.3		$	4.0

	December 31, 2011
	Derivative Assets			Derivative Liabilities
(in millions)	Balance Sheet Classification	Fair Value		Balance Sheet Classification	Fair Value
Current:
Commodity	Derivative assets	$	23.4	Derivative liabilities	$	37.7
Long-term:
Commodity	Derivative assets	10.5		Derivative liabilities	19.0
Total derivatives		$	33.9		$	56.7

Summary of (gains) losses before income taxes of derivative instruments recognized in the condensed statements of income (loss)

(in millions)		Year Ended December 31,
Description of (Gain) Loss	Location of (Gain) Loss Recognized in Earnings	2012			2011			2010
Commodity
Loss reclassified from AOCL into earnings (amortization of frozen amounts)	Oil and natural gas sales	$	10.7		$	60.9		$	18.4
(Gain) loss recognized in earnings (cash settlements and mark-to-market movements)	Realized and unrealized (gain) loss on derivatives, net	(64.6		)	(13.9		)	23.2
Interest rate
(Gain) loss reclassified from AOCL into earnings (amortization of frozen amounts)	Interest	$	(1.8	)	$	0.8		$	8.3
Loss recognized in earnings (cash settlements and mark-to-market movements)	Realized and unrealized (gain) loss on derivatives, net	—			—			8.6