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Shareholders' equity - Share Purchase Warrants and Deferred share unit plan (Details) - DSU plan
$ in Millions
12 Months Ended
Dec. 31, 2020
USD ($)
EquityInstruments
shares
Dec. 31, 2019
USD ($)
EquityInstruments
Stock-based payments    
Redeemed | shares 0  
Granted | EquityInstruments 14,285 18,448
DSU liability $ 14.9 $ 10.8
Mark-to-market adjustments $ 2.0 $ 2.8