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FINANCIAL RISK MANAGEMENT - Narrative (Details)
$ in Millions, $ in Millions
 12 Months Ended
Mar. 31, 2020
CAD ($)
Debt_tranche
shares
Mar. 31, 2019
CAD ($)
shares
May 19, 2020
USD ($)
Mar. 31, 2020
USD ($)
Mar. 31, 2019
USD ($)
Mar. 31, 2013
Foreign_currency_swap_agreement
Disclosure of maturity analysis for financial assets held for managing liquidity risk [line items]            
Accounts receivable, maximum amount available to sell       $ 300.0 $ 300.0  
Accounts receivable, amount sold to financial institution $ 333.1 $ 266.2        
Accounts receivable, amount sold to financial institution, fees 4.2 4.4        
Net income 318.9 [1],[2] 340.1        
Other comprehensive income (loss) $ 7.2 $ (55.3)        
Equity swap agreement, common shares (in shares) | shares 2,800,000 2,250,000        
Letters of credit and guarantees, net $ 189.6 $ 205.0        
Revolving Unsecured Term Credit Facility, September 2022            
Disclosure of maturity analysis for financial assets held for managing liquidity risk [line items]            
Line of credit facility, maximum borrowing capacity       $ 850.0 $ 550.0  
Foreign currency swap agreements            
Disclosure of maturity analysis for financial assets held for managing liquidity risk [line items]            
Debt, fixed interest rate 74.00% 83.00%   74.00% 83.00%  
Currency risk            
Disclosure of maturity analysis for financial assets held for managing liquidity risk [line items]            
Foreign currency strengthening change 5.00%     5.00%    
Interest rate risk            
Disclosure of maturity analysis for financial assets held for managing liquidity risk [line items]            
Sensitivity analysis to interest rate change 1.00%          
Net income $ 5.7 $ 4.1        
Other comprehensive income (loss) $ 0.0 0.0        
USD/CDN | Foreign currency swap agreements            
Disclosure of maturity analysis for financial assets held for managing liquidity risk [line items]            
Number of derivative agreements entered into | Foreign_currency_swap_agreement           2
Number of tranches relating to the multi-tranche private placement debt offering | Debt_tranche 2          
USD/CDN | Currency risk            
Disclosure of maturity analysis for financial assets held for managing liquidity risk [line items]            
Net income $ 4.8 3.0        
Other comprehensive income (loss) (19.6) (17.2)        
Multi-Tranche Private Placement Debt, Tranche One, December 2024 | USD/CDN | Foreign currency swap agreements            
Disclosure of maturity analysis for financial assets held for managing liquidity risk [line items]            
Derivative, notional amount 130.5 130.5   $ 127.0 $ 127.0  
Multi-Tranche Private Placement Debt, Tranche Two, December 2027 | USD/CDN | Foreign currency swap agreements            
Disclosure of maturity analysis for financial assets held for managing liquidity risk [line items]            
Derivative, notional amount $ 100.7 $ 100.7   98.0 98.0  
Hedges Of Net Investment In US Entities            
Disclosure of maturity analysis for financial assets held for managing liquidity risk [line items]            
Gross finance lease obligations       48.1 64.0  
Hedges Of Net Investment In US Entities | Senior Notes            
Disclosure of maturity analysis for financial assets held for managing liquidity risk [line items]            
Debt       $ 862.8 $ 822.8  
Cash and Liquidity Mitigation Measures            
Disclosure of maturity analysis for financial assets held for managing liquidity risk [line items]            
Accounts receivable, maximum amount available to sell     $ 400.0      
[1] Refer to Note 2 - Changes in accounting policies for the impact of adopting IFRS16.
[2] Refer to Note 2 - Changes in accounting policies for the impact of adopting IFRS16.