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Long-term Debt - Borrowing Availability (Details) - USD ($)
$ in Thousands
Dec. 31, 2015
Dec. 31, 2014
Debt Instrument [Line Items]    
Long-term Debt $ 2,082,368 $ 2,124,611
Letters of Credit Outstanding, Amount 33,200  
Credit Agreement [Member]    
Debt Instrument [Line Items]    
Credit agreement, unused borrowing capacity 296,800  
Revolving Credit Facility [Member]    
Debt Instrument [Line Items]    
Long-term Debt $ 20,000 $ 0