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Shareholders' equity - Common Shares (Details)
$ / shares in Units, $ in Thousands
 12 Months Ended
Feb. 19, 2016
$ / shares
shares
Dec. 31, 2017
USD ($)
$ / shares
shares
Dec. 31, 2016
USD ($)
shares
Shareholders' equity      
Proceeds from exercise of options | $   $ 10,100 $ 16,300
Proceeds from exercise of warrants | $   $ 356,400  
Common shares      
Shareholders' equity      
Shares issued | shares   185,930,331  
Shares, par value (dollars per share) | $ / shares   $ 0  
Exercise of stock options | shares   433,718 545,087
Proceeds from exercise of options | $   $ 10,100 $ 16,300
Exercise of stock warrants | shares   6,388,528 489
Proceeds from exercise of warrants | $   $ 356,400 $ 30
Vesting of restricted share units | shares   50,393 66,032
Common shares issued from dividend reinvestment plan | shares   575,553 588,182
Issue of equity through equity financing | shares 19,200,000    
Share price of equity financing (dollars per share) | $ / shares 47.85    
Net proceeds after deducting share issue costs and expenses | $   $ 883,500  
Share issue costs and expense | $   36,600  
Common shares | Share issue costs      
Shareholders' equity      
Deferred tax asset relating to share issue costs | $   $ 10,400  
Preferred shares      
Shareholders' equity      
Shares issued | shares   0