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Long-term Debt - Borrowing Availability (Details) - USD ($)
$ in Thousands
Apr. 30, 2017
Mar. 31, 2017
Dec. 31, 2016
Debt Instrument [Line Items]      
Long-term Debt   $ 2,369,646 $ 2,422,301
Letters of Credit and similar obligations outstanding, Amount   (33,200)  
Revolving Credit Facility [Member]      
Debt Instrument [Line Items]      
Credit agreement, unused borrowing capacity   616,800  
Revolving Credit Facility Due June 8, 2021 [Member] | Revolving Credit Facility [Member]      
Debt Instrument [Line Items]      
Long-term Debt   $ 35,000 $ 120,000
Revolving Credit Facility Due June 8, 2021 [Member] | Subsequent Event [Member] | Revolving Credit Facility [Member]      
Debt Instrument [Line Items]      
Long-term Debt $ 120,000