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Long-term debt - Schedule (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Disclosure of detailed information about borrowings [line items]    
Total long-term debt $ 1,502,271 $ 1,556,206
Less current maturities (55,905) (53,997)
Non-current portion of long-term debt 1,446,366 1,502,209
Debt discounts and deferred financing fees 17,800 17,800
Unsecured notes    
Disclosure of detailed information about borrowings [line items]    
Total long-term debt 1,188,163 1,186,424
3.25% due December 15, 2019    
Disclosure of detailed information about borrowings [line items]    
Total long-term debt $ 348,060 347,126
3.25% due December 15, 2019 | Fixed interest rate    
Disclosure of detailed information about borrowings [line items]    
Unsecured notes, interest rate 3.25%  
5.25% due March 1, 2022    
Disclosure of detailed information about borrowings [line items]    
Total long-term debt $ 248,072 247,685
5.25% due March 1, 2022 | Fixed interest rate    
Disclosure of detailed information about borrowings [line items]    
Unsecured notes, interest rate 5.25%  
4.25% due December 1, 2024    
Disclosure of detailed information about borrowings [line items]    
Total long-term debt $ 296,873 296,529
4.25% due December 1, 2024 | Fixed interest rate    
Disclosure of detailed information about borrowings [line items]    
Unsecured notes, interest rate 4.25%  
5.65% due December 1, 2044    
Disclosure of detailed information about borrowings [line items]    
Total long-term debt $ 295,158 295,084
5.65% due December 1, 2044 | Fixed interest rate    
Disclosure of detailed information about borrowings [line items]    
Unsecured notes, interest rate 5.65%  
Egypt limited recourse debt facilities    
Disclosure of detailed information about borrowings [line items]    
Total long-term debt $ 241,190 288,515
Other limited recourse debt facilities    
Disclosure of detailed information about borrowings [line items]    
Total long-term debt $ 72,918 $ 81,267