Employee benefit plans - Weighted average asset allocations (Details)	Dec. 31, 2020	Dec. 31, 2019
Defined Benefit Plan
Percentage of plan assets	100.00%	100.00%
Canadian equity
Defined Benefit Plan
Percentage of plan assets	31.00%	30.00%
U.S. equity
Defined Benefit Plan
Percentage of plan assets	14.00%	14.00%
International equity
Defined Benefit Plan
Percentage of plan assets	17.00%	14.00%
Canadian fixed income
Defined Benefit Plan
Percentage of plan assets	38.00%	39.00%
Real estate equities
Defined Benefit Plan
Percentage of plan assets		3.00%