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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, 2019
Dec. 31, 2018
Employee Benefit Plan    
Investment funds    
Money market 2.00% 2.00%
Canadian fixed income 98.00% 98.00%
Total 100.00% 100.00%
SERP    
Investment funds    
Money market 7.00% 1.00%
Canadian fixed income 0.00% 6.00%
Other 93.00% 49.00%
Total 100.00% 100.00%
SERP | Canada    
Investment funds    
Equities 0.00% 22.00%
SERP | UNITED STATES    
Investment funds    
Equities 0.00% 10.00%
SERP | International equities    
Investment funds    
Equities 0.00% 12.00%