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Capital Stock (Narrative - Share Based Compensation) (Details) - USD ($)
shares in Millions, $ in Millions
12 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Mar. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Excess tax deficiencies on equity-based compensation awards $ 14.9    
Excess tax benefits on equity-based compensation awards   $ 5.2 $ 0.0
Stock Options      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Exercised, total intrinsic value $ 5.3 $ 36.9 $ 30.0
Shares cancelled to fund withholding tax obligations on option exercises (in shares) 0.1 0.1 0.8
Restricted Stock and Restricted Share Units (RSUs)      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Fair value of restricted stock and restricted share units vested $ 33.7 $ 41.6 $ 60.7
Shares withheld for tax purposes upon vesting of restricted share units (in shares) 0.5 0.7 1.0
Stock Compensation Plan      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Other share-based compensation granted in period, fair value $ 2.3 $ 0.8 $ 1.1
Shares issued, net of shares for tax withholdings (in shares) 0.1    
Stock Compensation Plan | Maximum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Shares issued, net of shares for tax withholdings (in shares)   0.1 0.1