Debt Debt, Textuals (Details) (USD $)	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Dec. 31, 2011
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	6.61%		6.69%
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate	89.00%		89.00%
Line of Credit Facility, Maximum Borrowing Capacity	$ 500,000,000		$ 400,000,000
Letters of Credit, Maximum Borrowing Capacity			175,000,000
Line of Credit Facility, Amount Outstanding	65,000,000		65,000,000
Letters of Credit Outstanding, Amount	32,700,000
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage	0.35%
Line of Credit Facility, Remaining Borrowing Capacity	402,300,000
Repayments of Long-term Capital Lease Obligations	6,084,000	5,639,000
Repayments of Debt		5,200,000
Gains (Losses) on Extinguishment of Debt	0	(314,000)
Capital Lease Obligations Incurred	38,300,000	8,800,000
February 2013 [Member]
Debt Instrument [Line Items]
Repayments of Debt		200,000
Debt Instrument, Interest Rate, Stated Percentage		0.00%
April 2015 [Member]
Debt Instrument [Line Items]
Repayments of Debt		$ 4,700,000
Debt Instrument, Interest Rate, Stated Percentage		6.75%