Derivative Commodity Instruments - Offsetting Assets and Liabilities (Details) (USD $)	Sep. 30, 2014	Dec. 31, 2013
Derivative assets
Gross Amounts Recognized at Fair Value	$ 43,620,000	$ 44,215,000
Gross Amounts Offset in the Balance Sheets	(7,667,000)	(21,313,000)
Total assets	35,953,000	22,902,000
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Fair Value Presented in the Balance Sheets	35,953,000	22,902,000
Derivative assets
Gross Amounts Recognized at Fair Value	9,115,000	51,615,000
Gross Amounts Offset in the Balance Sheets	(7,667,000)	(21,313,000)
Net Amount Presented in the Balance Sheets	1,448,000	30,302,000
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Fair Value Presented in the Balance Sheets	1,448,000	30,302,000
Total Derivatives	34,505,000	(7,400,000)
Net deferred tax liability	2,200,000	8,200,000
Current Assets
Derivative assets
Gross Amounts Recognized at Fair Value	37,039,000	36,223,000
Gross Amounts Offset in the Balance Sheets	(6,844,000)	(18,760,000)
Total assets	30,195,000	17,463,000
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Fair Value Presented in the Balance Sheets	30,195,000	17,463,000
Noncurrent Assets
Derivative assets
Gross Amounts Recognized at Fair Value	6,581,000	7,992,000
Gross Amounts Offset in the Balance Sheets	(823,000)	(2,553,000)
Total assets	5,758,000	5,439,000
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Fair Value Presented in the Balance Sheets	5,758,000	5,439,000
Current Liabilities
Derivative assets
Gross Amounts Recognized at Fair Value	7,652,000	49,062,000
Gross Amounts Offset in the Balance Sheets	(6,844,000)	(18,760,000)
Net Amount Presented in the Balance Sheets	808,000	30,302,000
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Fair Value Presented in the Balance Sheets	808,000	30,302,000
Noncurrent Liabilities
Derivative assets
Gross Amounts Recognized at Fair Value	1,463,000	2,553,000
Gross Amounts Offset in the Balance Sheets	(823,000)	(2,553,000)
Net Amount Presented in the Balance Sheets	640,000	0
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Fair Value Presented in the Balance Sheets	$ 640,000	$ 0