Derivative Instruments (Details 4) (USD $)	Mar. 31, 2013	Dec. 31, 2012
Derivative [Line Items]
Derivative assets, current	$ 7,622,000	$ 14,661,000
Derivative liabilities, current	5,057,000	1,111,000
Derivative assets, noncurrent	17,491,000	10,891,000
Derivative liabilities, noncurrent	0	1,239,000
Commodity Contract
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	36,400,000	27,300,000
Derivative Asset, Fair Value, Gross Asset Offset	(11,300,000)	(1,700,000)
Derivative Liability, Fair Value, Gross Liability	16,400,000	4,000,000
Derivative Liability, Fair Value, Gross Liability Offset	(11,300,000)	(1,700,000)
Derivative assets, current	7,622,000	14,661,000
Derivative liabilities, current	5,057,000	1,111,000
Derivative assets, noncurrent	17,491,000	10,891,000
Derivative liabilities, noncurrent	0	1,239,000
Total derivative assets	25,100,000	25,600,000
Total derivative liabilities	5,100,000	2,300,000
Current | Commodity Contract
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	12,400,000	16,400,000
Derivative Asset, Fair Value, Gross Asset Offset	(4,800,000)	(1,700,000)
Derivative Liability, Fair Value, Gross Liability	9,900,000	2,800,000
Derivative Liability, Fair Value, Gross Liability Offset	(4,800,000)	(1,700,000)
Long-term | Commodity Contract
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	24,000,000	10,900,000
Derivative Asset, Fair Value, Gross Asset Offset	(6,500,000)	0
Derivative Liability, Fair Value, Gross Liability	6,500,000	1,200,000
Derivative Liability, Fair Value, Gross Liability Offset	$ (6,500,000)	$ 0