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Debt Narratives (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 30, 2016
Dec. 31, 2016
Debt Instrument [Line Items]          
Debt, Weighted Average Interest Rate 4.80%   4.80%   4.68%
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate 62.00%   62.00%   63.00%
Interest Paid     $ 79,900 $ 80,200  
Letters of Credit Outstanding, Amount $ 34,300   $ 34,300    
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage     0.30%    
Line of Credit Facility, Remaining Borrowing Capacity 205,700   $ 205,700    
Line of Credit Facility, Increase (Decrease), Net     110,000    
Repayments of Debt     17,600    
Repayments of Long-term Debt     17,570 18,835  
Gain (Loss) on Extinguishment of Debt 0 $ (21,898) 0 (22,479)  
Repayments of Long-term Capital Lease Obligations     30,419 $ 16,907  
March 2021          
Debt Instrument [Line Items]          
Line of Credit Facility, Amount Outstanding 460,000   460,000   $ 350,000
Letters of Credit Outstanding, Amount 460,000   460,000    
Term Loan [Member]          
Debt Instrument [Line Items]          
Repayments of Debt     17,500    
Unsecured Debt | May 2024          
Debt Instrument [Line Items]          
Senior Notes $ 850,000   $ 850,000   $ 850,000