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Debt Narratives (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Dec. 31, 2016
Debt Instrument [Line Items]      
Debt, Weighted Average Interest Rate 4.80%   4.70%
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate 63.00%    
Interest Paid $ 20,000 $ 15,900  
Letters of Credit Outstanding, Amount $ 32,700    
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage 0.30%    
Line of Credit Facility, Remaining Borrowing Capacity $ 282,300    
Repayments of Debt 8,800    
Repayments of Long-term Debt 8,787 10,054  
Gain (Loss) on Extinguishment of Debt 0 (581)  
Repayments of Long-term Capital Lease Obligations 21,055 $ 8,156  
March 2021      
Debt Instrument [Line Items]      
Line of Credit Facility, Amount Outstanding 385,000   $ 350,000
Letters of Credit Outstanding, Amount 385,000    
January 2017 [Member]      
Debt Instrument [Line Items]      
Line of Credit Facility, Increase (Decrease), Net 25,000    
March 2017 [Member]      
Debt Instrument [Line Items]      
Line of Credit Facility, Increase (Decrease), Net 10,000    
Unsecured Debt | May 2024      
Debt Instrument [Line Items]      
Senior Notes $ 850,000   $ 850,000