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FINANCIAL RISK MANAGEMENT - Forward Foreign Currency Contracts (Details) - Currency risk
$ in Millions
 12 Months Ended
Mar. 31, 2018
CAD ($)
Mar. 31, 2017
CAD ($)
Disclosure of risk management strategy related to hedge accounting [line items]    
Risk exposure associated with instruments sharing characteristic $ 1,351.4 $ 1,380.3
USD/CDN | No later than 1 year    
Disclosure of risk management strategy related to hedge accounting [line items]    
Risk exposure associated with instruments sharing characteristic $ 572.2 $ 567.7
Average foreign exchange rate 0.79 0.74
USD/CDN | Between 1 and 3 years    
Disclosure of risk management strategy related to hedge accounting [line items]    
Risk exposure associated with instruments sharing characteristic $ 131.6 $ 158.5
Average foreign exchange rate 0.78 0.77
USD/CDN | Between 3 and 5 years    
Disclosure of risk management strategy related to hedge accounting [line items]    
Risk exposure associated with instruments sharing characteristic $ 20.2 $ 8.6
Average foreign exchange rate 0.80 0.77
CDN/EUR | No later than 1 year    
Disclosure of risk management strategy related to hedge accounting [line items]    
Risk exposure associated with instruments sharing characteristic $ 40.1 $ 33.3
Average foreign exchange rate 1.57 1.45
CDN/EUR | Between 1 and 3 years    
Disclosure of risk management strategy related to hedge accounting [line items]    
Risk exposure associated with instruments sharing characteristic $ 0.0 $ 15.2
Average foreign exchange rate 0.00 1.45
EUR/CDN | No later than 1 year    
Disclosure of risk management strategy related to hedge accounting [line items]    
Risk exposure associated with instruments sharing characteristic $ 125.6 $ 79.7
Average foreign exchange rate 0.65 0.69
EUR/CDN | Between 1 and 3 years    
Disclosure of risk management strategy related to hedge accounting [line items]    
Risk exposure associated with instruments sharing characteristic $ 2.5 $ 4.9
Average foreign exchange rate 0.63 0.67
EUR/CDN | Between 3 and 5 years    
Disclosure of risk management strategy related to hedge accounting [line items]    
Risk exposure associated with instruments sharing characteristic $ 0.2 $ 0.0
Average foreign exchange rate 0.59 0.00
GBP/CDN | No later than 1 year    
Disclosure of risk management strategy related to hedge accounting [line items]    
Risk exposure associated with instruments sharing characteristic $ 72.4 $ 116.3
Average foreign exchange rate 0.56 0.58
GBP/CDN | Between 1 and 3 years    
Disclosure of risk management strategy related to hedge accounting [line items]    
Risk exposure associated with instruments sharing characteristic $ 19.4 $ 9.9
Average foreign exchange rate 0.57 0.55
GBP/CDN | Between 3 and 5 years    
Disclosure of risk management strategy related to hedge accounting [line items]    
Risk exposure associated with instruments sharing characteristic $ 0.0 $ 0.1
Average foreign exchange rate 0.00 0.51
CDN/GBP | No later than 1 year    
Disclosure of risk management strategy related to hedge accounting [line items]    
Risk exposure associated with instruments sharing characteristic $ 33.4 $ 25.7
Average foreign exchange rate 1.80 1.73
CDN/GBP | Between 1 and 3 years    
Disclosure of risk management strategy related to hedge accounting [line items]    
Risk exposure associated with instruments sharing characteristic $ 0.0 $ 3.1
Average foreign exchange rate 0.00 1.74
CDN/USD | No later than 1 year    
Disclosure of risk management strategy related to hedge accounting [line items]    
Risk exposure associated with instruments sharing characteristic $ 132.3 $ 130.0
Average foreign exchange rate 1.28 1.32
CDN/USD | Between 1 and 3 years    
Disclosure of risk management strategy related to hedge accounting [line items]    
Risk exposure associated with instruments sharing characteristic $ 5.8 $ 8.7
Average foreign exchange rate 1.29 1.33
GBP/USD | No later than 1 year    
Disclosure of risk management strategy related to hedge accounting [line items]    
Risk exposure associated with instruments sharing characteristic $ 31.0 $ 76.8
Average foreign exchange rate 0.71 0.76
GBP/USD | Between 1 and 3 years    
Disclosure of risk management strategy related to hedge accounting [line items]    
Risk exposure associated with instruments sharing characteristic $ 12.8 $ 11.1
Average foreign exchange rate 0.76 0.69
Other Currencies | No later than 1 year    
Disclosure of risk management strategy related to hedge accounting [line items]    
Risk exposure associated with instruments sharing characteristic $ 139.6 $ 111.1
Average foreign exchange rate 0.00 0.00
Other Currencies | Between 1 and 3 years    
Disclosure of risk management strategy related to hedge accounting [line items]    
Risk exposure associated with instruments sharing characteristic $ 12.3 $ 19.6
Average foreign exchange rate 0.00 0.00