Debt Debt, Textuals (Details) (USD $)	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Dec. 31, 2013
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	5.38%		5.25%
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate	72.00%		76.00%
Line of Credit Facility, Maximum Borrowing Capacity	$ 500,000,000
Letters of Credit, Maximum Borrowing Capacity	175,000,000
Letters of Credit Outstanding, Amount	31,600,000
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage	0.40%
Line of Credit Facility, Remaining Borrowing Capacity	328,400,000
Repayments of Debt and Capital Lease Obligations	181,900,000	11,400,000
Repayments of Long-term Capital Lease Obligations	7,231,000	6,468,000
Repayments of Long-term Debt	7,579,000	4,948,000
Capital Lease Obligations Incurred	10,700,000	14,900,000
July 2014 [Member]
Debt Instrument [Line Items]
Repayments of Debt	86,300,000
Repayments of Long-term Debt	21,700,000
July 2016 [Member]
Debt Instrument [Line Items]
Repayments of Debt	45,000,000
Repayments of Long-term Debt	14,200,000
July 2018 [Member]
Debt Instrument [Line Items]
Repayments of Debt	$ 7,500,000
February 2013 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	7.875%