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Employee benefit plans - Summary of defined benefit plans' weighted average asset allocation percentages by asset category (Details)
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Employee benefit plans    
Investment funds    
Money market 0.00% 2.00%
Canadian fixed income 0.00% 98.00%
Other 0.00% 0.00%
Total 0.00% 100.00%
Employee benefit plans | Canada    
Investment funds    
Equities 0.00% 0.00%
Employee benefit plans | UNITED STATES    
Investment funds    
Equities 0.00% 0.00%
Employee benefit plans | Other jurisdictions    
Investment funds    
Equities 0.00% 0.00%
SERP    
Investment funds    
Money market 80.00% 7.00%
Canadian fixed income 0.00% 0.00%
Other 20.00% 93.00%
Total 100.00% 100.00%
SERP | Canada    
Investment funds    
Equities 0.00% 0.00%
SERP | UNITED STATES    
Investment funds    
Equities 0.00% 0.00%
SERP | Other jurisdictions    
Investment funds    
Equities 0.00% 0.00%