Debt Debt, Textuals (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	6.79%		6.79%		6.80%
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate	92.20%		92.20%		92.50%
Line of Credit Facility, Maximum Borrowing Capacity	$ 500,000,000		$ 500,000,000		$ 400,000,000
Letters of Credit, Maximum Borrowing Capacity	175,000,000		175,000,000
Letters of Credit Outstanding, Amount	33,300,000		33,300,000
Line of Credit Facility, Remaining Borrowing Capacity	466,700,000		466,700,000
Repayments of Debt			28,100,000	30,000,000
Gains (Losses) on Extinguishment of Debt	(1,835,000)	(291,000)	(2,149,000)	(291,000)
Capital Lease Obligations Incurred			19,100,000	11,700,000
February 2013 [Member]
Debt Instrument [Line Items]
Repayments of Debt			3,700,000
Debt Instrument, Interest Rate, Stated Percentage	7.88%		7.88%
April 2015 [Member]
Debt Instrument [Line Items]
Repayments of Debt			15,600,000	3,000,000
Debt Instrument, Interest Rate, Stated Percentage	6.75%	6.75%	6.75%	6.75%
April 2016 [Member]
Debt Instrument [Line Items]
Repayments of Debt			$ 7,000,000	$ 20,200,000
Debt Instrument, Interest Rate, Stated Percentage	6.75%	6.75%	6.75%	6.75%