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EMPLOYEE BENEFITS OBLIGATIONS - Expected Employer Contributions (Details)
$ in Millions
 12 Months Ended
Mar. 31, 2020
CAD ($)
Funded  
Disclosure of defined benefit plans [line items]  
Estimate of contributions expected to be paid to plan for next annual reporting period $ 24.8
Unfunded  
Disclosure of defined benefit plans [line items]  
Estimate of contributions expected to be paid to plan for next annual reporting period 3.9
Canada | Funded  
Disclosure of defined benefit plans [line items]  
Estimate of contributions expected to be paid to plan for next annual reporting period 21.7
Canada | Unfunded  
Disclosure of defined benefit plans [line items]  
Estimate of contributions expected to be paid to plan for next annual reporting period 3.3
Foreign | Funded  
Disclosure of defined benefit plans [line items]  
Estimate of contributions expected to be paid to plan for next annual reporting period 3.1
Foreign | Unfunded  
Disclosure of defined benefit plans [line items]  
Estimate of contributions expected to be paid to plan for next annual reporting period $ 0.6