INCOME TAXES - Components of the Provision for the Income Tax Expense (Details) - CAD ($) $ in Millions	12 Months Ended
Mar. 31, 2019	Mar. 31, 2018
Income Tax [Abstract]
Current period	$ 69.9	$ 53.8
Adjustment for prior years	12.8	11.0
Tax benefit not previously recognized used to reduce the deferred tax expense	(36.1)	(18.7)
Impact of change in income tax rates on deferred income taxes	(1.3)	(31.2)
Origination and reversal of temporary differences	14.3	16.0
Income tax expense	$ 59.6	$ 30.9

Income Tax [Abstract]

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Adjustments of tax expense (income) recognised in the period for current tax of prior periods.

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The amount of income taxes payable (recoverable) in respect of the taxable profit (tax loss) for a period.

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The amount of deferred tax expense or income relating to the creation or reversal of temporary differences. [Refer: Temporary differences [member]; Deferred tax expense (income)]

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The amount of deferred tax expense or income relating to tax rate changes or the imposition of new taxes. [Refer: Deferred tax expense (income)]

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The aggregate amount included in the determination of profit (loss) for the period in respect of current tax and deferred tax. [Refer: Current tax expense (income); Deferred tax expense (income)]

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The amount of benefit arising from a previously unrecognised tax loss, tax credit or temporary difference of a prior period that is used to reduce deferred tax expense. [Refer: Deferred tax expense (income); Temporary differences [member]; Unused tax losses [member]; Unused tax credits [member]]

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