Debt (Details) (USD $)			12 Months Ended			12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012 Line of Credit-short term day	Dec. 31, 2012 Senior secured revolving credit agreement (Credit Agreement - long term) Redetermination	Apr. 13, 2012 Senior secured revolving credit agreement (Credit Agreement - long term)	Dec. 31, 2012 Low End of the Range Senior secured revolving credit agreement (Credit Agreement - long term)	Dec. 31, 2012 Low End of the Range Senior secured revolving credit agreement (Credit Agreement - long term) LIBOR	Dec. 31, 2012 Low End of the Range Senior secured revolving credit agreement (Credit Agreement - long term) Debt Instrument Variable Rate Prime	Dec. 31, 2012 High End of the Range Senior secured revolving credit agreement (Credit Agreement - long term)	Dec. 31, 2012 High End of the Range Senior secured revolving credit agreement (Credit Agreement - long term) LIBOR	Dec. 31, 2012 High End of the Range Senior secured revolving credit agreement (Credit Agreement - long term) Debt Instrument Variable Rate Prime
Debt Instrument
Line of Credit Facility, Maximum Borrowing Capacity					$ 1,400,000,000
Line of credit, current borrowing capacity			40,000,000
Line of credit, maximum number of days which borrowings may be outstanding (in days)			30
Lender commitments				1,200,000,000
Loans Payable Basis Spread On Variable Rate							1.50%	0.50%		2.50%	1.50%
Outstanding borrowing under credit facility	562,900,000	531,500,000
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				$ 613,900,000
Debt instrument covenant consolidated interest coverage ratio numerator (as a ratio)				2.75
Debt instrument covenant consolidated interest coverage ratio denominator (as a ratio)				1.0
Debt instrument covenant minimum current ratio numerator (as a ratio)				1.0
Debt instrument covenant minimum current ratio denominator (as a ratio)				1.0
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage						0.35%			0.50%