Debt (Details) (USD $)	12 Months Ended		12 Months Ended		0 Months Ended	3 Months Ended	12 Months Ended		12 Months Ended
	Dec. 31, 2011 Redetermination	Dec. 31, 2010	Dec. 31, 2011 Line of Credit-short term day	Dec. 31, 2010 Line of Credit-short term day	Dec. 31, 2011 Senior secured revolving credit agreement (Credit Agreement - long term) ratio Lenders	Dec. 31, 2011 Senior secured revolving credit agreement (Credit Agreement - long term) Lenders	Dec. 31, 2011 Senior secured revolving credit agreement (Credit Agreement - long term) Lenders	Dec. 31, 2010 Senior secured revolving credit agreement (Credit Agreement - long term)	Dec. 31, 2011 Senior secured revolving credit agreement (Credit Agreement - long term) LIBOR	Dec. 31, 2011 Senior secured revolving credit agreement (Credit Agreement - long term) Prime
Debt obligations
Line of credit, current borrowing capacity			$ 40,000,000	$ 40,000,000
Line of credit, maximum number of days which borrowings may be outstanding (in days)			30	30
Short-term Bank Loans and Notes Payable	0	5,300,000	0	5,300,000
Line of credit, margin rate over LIBOR (as a percent)			1.40%
Weighted average interest rate on outstanding borrowings (as a percent)				1.70%
Credit agreement maximum borrowing base after amendment					1,400,000,000	1,400,000,000	1,400,000,000
Lender commitments					1,200,000,000	1,200,000,000	1,200,000,000
Debt instrument basis spread on variable rate low end of range (as a percent)									1.50%	0.50%
Debt Instrument basis spread on variable rate high end of range (as a percent)									2.50%	1.50%
Annual commitment fee on the unused portion of the credit agreement low end of range (as a percent)							0.35%
Annual commitment fee on the unused portion of the credit agreement high end of range (as a percent)							0.50%
Fee paid for additional lender commitments							2,200,000
Write-off of debt issuance costs						500,000
Number of lenders that did not renew commitment to credit agreement (in lenders)					1	1	1
Outstanding borrowing under credit facility	531,500,000	170,000,000						170,000,000
Line of credit facility, borrowing base number of additional redeterminations	1
Letters of credit that have been issued under the facility	23,200,000
Remaining borrowing capacity available under the agreement	$ 645,300,000
Debt instrument covenant consolidated interest coverage ratio numerator (as a ratio)					2.75
Debt instrument covenant consolidated interest coverage ratio denominator (as a ratio)					1.00
Debt instrument covenant consolidated minimum current ratio numerator (as a ratio)					1.0
Debt instrument covenant consolidated minimum current ratio denominator (as a ratio)					1.0