Debt Narratives (Details) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	5.21%		5.21%
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate	75.00%		75.00%
Line of Credit Facility, Maximum Borrowing Capacity	$ 500,000,000
Long-term Line of Credit	355,000,000		370,000,000
Letters of Credit, Maximum Borrowing Capacity	175,000,000
Line of Credit Facility, Amount Outstanding	250,000,000		235,000,000
Letters of Credit Outstanding, Amount	31,600,000
Repayments of Debt and Capital Lease Obligations	7,400,000	7,200,000
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage	0.35%
Line of Credit Facility, Remaining Borrowing Capacity	218,400,000
Line of Credit Facility, Increase (Decrease), Net	15,000,000
Early Repayment of Senior Debt	0	167,093,000
Repayments of Long-term Capital Lease Obligations	7,380,000	7,231,000
Repayments of Debt	15,000,000	7,500,000
Repayments of Long-term Debt	15,071,000	7,579,000
Capital Lease Obligations Incurred	$ 7,300,000	$ 10,700,000