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Pension Plans and Other Post-Retirement Benefits - Schedule of Actuarial Assumptions (Details) - year
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Canada    
To determine defined benefit obligation at end of year:    
Discount rate for pension plans 2.70% 3.00%
Rate of compensation increase 2.80% 3.00%
To determine net benefit expense for year:    
Discount rate for pension plans 3.00% 3.60%
Rate of compensation increase 3.00% 3.00%
Health care trend rates:    
Initial health care trend rate 5.30% 5.36%
Ultimate health care trend rate 4.00% 4.50%
Year ultimate health care trend rate reached 2040 2030
Mortality rates:    
Average duration (in years) of pension obligation 17 years 2 months 12 days 16 years 8 months 12 days
United Kingdom    
To determine defined benefit obligation at end of year:    
Discount rate for pension plans 1.30% 2.00%
Pension increases 2.95% 2.95%
To determine net benefit expense for year:    
Discount rate for pension plans 2.00% 2.85%
Pension increases 2.95% 3.15%
Mortality rates:    
Average duration (in years) of pension obligation 17 years 8 months 12 days 16 years 7 months 6 days
Pension increases 2.95% 2.95%
Pension increases 2.95% 3.15%
United States    
To determine defined benefit obligation at end of year:    
Discount rate for pension plans 2.65% 3.45%
To determine net benefit expense for year:    
Discount rate for pension plans 3.45% 4.40%
Health care trend rates:    
Initial health care trend rate 650.00% 6.50%
Ultimate health care trend rate 500.00% 5.00%
Year ultimate health care trend rate reached 2025 2025
Mortality rates:    
Average duration (in years) of pension obligation 12 years 8 months 12 days 13 years 4 months 24 days
Bottom of range | Canada    
To determine defined benefit obligation at end of year:    
Pension increases 0.00% 0.00%
To determine net benefit expense for year:    
Pension increases 0.00% 0.00%
Mortality rates:    
Pension increases 0.00% 0.00%
Pension increases 0.00% 0.00%
Top of range | Canada    
To determine defined benefit obligation at end of year:    
Pension increases 0.05% 0.15%
To determine net benefit expense for year:    
Pension increases 0.015% 0.15%
Mortality rates:    
Pension increases 0.05% 0.15%
Pension increases 0.015% 0.15%
Male - Individuals currently at age 65 | Canada    
Mortality rates:    
Life expectancy (in years) for individuals 23 23
Male - Individuals currently at age 65 | United Kingdom    
Mortality rates:    
Life expectancy (in years) for individuals 23 23
Male - Individuals currently at age 65 | United States    
Mortality rates:    
Life expectancy (in years) for individuals 22 22
Female - Individuals currently at age 65 | Canada    
Mortality rates:    
Life expectancy (in years) for individuals 25 25
Female - Individuals currently at age 65 | United Kingdom    
Mortality rates:    
Life expectancy (in years) for individuals 25 24
Female - Individuals currently at age 65 | United States    
Mortality rates:    
Life expectancy (in years) for individuals 23 24
Male - At 65 for individuals currently at age 45 | Canada    
Mortality rates:    
Life expectancy (in years) for individuals 24 24
Male - At 65 for individuals currently at age 45 | United Kingdom    
Mortality rates:    
Life expectancy (in years) for individuals 25 25
Male - At 65 for individuals currently at age 45 | United States    
Mortality rates:    
Life expectancy (in years) for individuals 23 24
Female - At 65 for individuals currently at age 45 | Canada    
Mortality rates:    
Life expectancy (in years) for individuals 26 26
Female - At 65 for individuals currently at age 45 | United Kingdom    
Mortality rates:    
Life expectancy (in years) for individuals 27 27
Female - At 65 for individuals currently at age 45 | United States    
Mortality rates:    
Life expectancy (in years) for individuals 25 25