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Long-term debt - Maturity (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Disclosure of maturity analysis for non-derivative financial liabilities [line items]    
Long-term debt $ 1,502,271 $ 1,556,206
Limited recourse debt facilities    
Disclosure of maturity analysis for non-derivative financial liabilities [line items]    
Long-term debt 314,108 369,782
Unsecured notes    
Disclosure of maturity analysis for non-derivative financial liabilities [line items]    
Long-term debt 1,188,163 $ 1,186,424
Carrying amount    
Disclosure of maturity analysis for non-derivative financial liabilities [line items]    
Long-term debt 1,520,028  
Carrying amount | 2018    
Disclosure of maturity analysis for non-derivative financial liabilities [line items]    
Long-term debt 57,072  
Carrying amount | 2019    
Disclosure of maturity analysis for non-derivative financial liabilities [line items]    
Long-term debt 410,100  
Carrying amount | 2020    
Disclosure of maturity analysis for non-derivative financial liabilities [line items]    
Long-term debt 62,115  
Carrying amount | 2021    
Disclosure of maturity analysis for non-derivative financial liabilities [line items]    
Long-term debt 109,462  
Carrying amount | 2022    
Disclosure of maturity analysis for non-derivative financial liabilities [line items]    
Long-term debt 281,279  
Carrying amount | Thereafter    
Disclosure of maturity analysis for non-derivative financial liabilities [line items]    
Long-term debt 600,000  
Carrying amount | Limited recourse debt facilities    
Disclosure of maturity analysis for non-derivative financial liabilities [line items]    
Long-term debt 320,028  
Carrying amount | Limited recourse debt facilities | 2018    
Disclosure of maturity analysis for non-derivative financial liabilities [line items]    
Long-term debt 57,072  
Carrying amount | Limited recourse debt facilities | 2019    
Disclosure of maturity analysis for non-derivative financial liabilities [line items]    
Long-term debt 60,100  
Carrying amount | Limited recourse debt facilities | 2020    
Disclosure of maturity analysis for non-derivative financial liabilities [line items]    
Long-term debt 62,115  
Carrying amount | Limited recourse debt facilities | 2021    
Disclosure of maturity analysis for non-derivative financial liabilities [line items]    
Long-term debt 109,462  
Carrying amount | Limited recourse debt facilities | 2022    
Disclosure of maturity analysis for non-derivative financial liabilities [line items]    
Long-term debt 31,279  
Carrying amount | Limited recourse debt facilities | Thereafter    
Disclosure of maturity analysis for non-derivative financial liabilities [line items]    
Long-term debt 0  
Carrying amount | Unsecured notes    
Disclosure of maturity analysis for non-derivative financial liabilities [line items]    
Long-term debt 1,200,000  
Carrying amount | Unsecured notes | 2018    
Disclosure of maturity analysis for non-derivative financial liabilities [line items]    
Long-term debt 0  
Carrying amount | Unsecured notes | 2019    
Disclosure of maturity analysis for non-derivative financial liabilities [line items]    
Long-term debt 350,000  
Carrying amount | Unsecured notes | 2020    
Disclosure of maturity analysis for non-derivative financial liabilities [line items]    
Long-term debt 0  
Carrying amount | Unsecured notes | 2021    
Disclosure of maturity analysis for non-derivative financial liabilities [line items]    
Long-term debt 0  
Carrying amount | Unsecured notes | 2022    
Disclosure of maturity analysis for non-derivative financial liabilities [line items]    
Long-term debt 250,000  
Carrying amount | Unsecured notes | Thereafter    
Disclosure of maturity analysis for non-derivative financial liabilities [line items]    
Long-term debt $ 600,000