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Debt Narratives (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 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     $ 101,700 $ 98,800  
Letters of Credit Outstanding, Amount $ 33,400   $ 33,400    
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage     0.30%    
Line of Credit Facility, Remaining Borrowing Capacity 196,600   $ 196,600    
Line of Credit Facility, Increase (Decrease), Net     120,000    
Repayments of Debt     26,400    
Repayments of Long-term Debt     26,376 27,632  
Gain (Loss) on Extinguishment of Debt 0 $ (25) 0 (22,503)  
Repayments of Long-term Capital Lease Obligations     40,509 $ 25,220  
March 2021          
Debt Instrument [Line Items]          
Line of Credit Facility, Amount Outstanding 470,000   470,000   $ 350,000
Letters of Credit Outstanding, Amount 470,000   470,000    
Term Loan [Member]          
Debt Instrument [Line Items]          
Repayments of Debt     26,300    
Unsecured Debt | May 2024          
Debt Instrument [Line Items]          
Senior Notes $ 850,000   $ 850,000   $ 850,000