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Long-term debt outstanding (Details 3) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Long term debt outstanding [Abstract]    
Long-term debt $ 1,714,853 $ 1,790,159
Long-term debt due within one year 148,499 43,598
Long-term debt $ 1,566,354 1,746,561
Senior notes [Member]    
Debt instrument [Line Items]    
Weighted average interest rate 4.71%  
Long term debt outstanding [Abstract]    
Long-term debt $ 1,499,916 1,437,831
Commercial paper [Member]    
Debt instrument [Line Items]    
Weighted average interest rate 1.72%  
Long term debt outstanding [Abstract]    
Long-term debt $ 88,350 262,000
Medium-term notes [Member]    
Debt instrument [Line Items]    
Weighted average interest rate 6.68%  
Long term debt outstanding [Abstract]    
Long-term debt $ 50,000 50,000
Other notes [Member]    
Debt instrument [Line Items]    
Weighted average interest rate 5.24%  
Long term debt outstanding [Abstract]    
Long-term debt $ 24,982 24,471
Revolving credit facility [Member]    
Debt instrument [Line Items]    
Weighted average interest rate 3.71%  
Long term debt outstanding [Abstract]    
Long-term debt $ 57,300 21,793
Long-term debt [Member]    
Long term debt outstanding [Abstract]    
Unamortized debt issuance costs 5,694 5,832
Discount $ 1 $ 104