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Commitments and contingencies (Details)
$ in Thousands
 12 Months Ended
Nov. 13, 2018
Dec. 31, 2018
USD ($)
claim
Dec. 31, 2018
CAD ($)
Dec. 31, 2017
USD ($)
Disclosure of other provisions [line items]        
Capital commitments   $ 2,911,000   $ 3,049,000
Number of claims remaining | claim   1    
Royalty related to UTC, revenue obligations        
Disclosure of other provisions [line items]        
Expiration period   15 years    
Royalty obligation, percentage   2.00%    
Maximum royalty expense   $ 4,613,000 $ 5,351  
Royalty expense   $ 0    
Royalty of other revenue obligations        
Disclosure of other provisions [line items]        
Royalty obligation, percentage   2.00%    
Maximum royalty expense   $ 1,896,000 $ 2,200  
Royalty expense   $ 0    
Weichai Ballard JV        
Disclosure of other provisions [line items]        
Cash contribution term 4 years 4 years