EMPLOYEE BENEFITS OBLIGATIONS - Movement In Defined Benefit Pension Obligation (Details) - CAD ($) $ in Millions	12 Months Ended
Mar. 31, 2018	Mar. 31, 2017
Disclosure of net defined benefit liability (asset) [line items]
Net defined benefit liability (asset)	$ 157.7
Actuarial loss (gain) arising from:
Net defined benefit liability (asset)	200.6	$ 157.7
Foreign
Disclosure of net defined benefit liability (asset) [line items]
Interest cost/Interest income		(1.1)
Funded
Disclosure of net defined benefit liability (asset) [line items]
Net defined benefit liability (asset)	78.6
Current service cost	23.8	23.2
Interest cost/Interest income	(15.2)	(14.7)
Actuarial loss (gain) arising from:
Net defined benefit liability (asset)	114.8	78.6
Funded \| Present value of defined benefit obligation
Disclosure of net defined benefit liability (asset) [line items]
Net defined benefit liability (asset)	541.3	521.2
Current service cost	23.8	23.2
Interest cost/Interest income	17.9	17.8
Actuarial loss (gain) arising from:
Experience adjustments	0.5	1.7
Economic assumptions	30.1	15.3
Demographic assumptions	4.8	(18.6)
Employee contributions	6.2	5.9
Pension benefits paid	(19.0)	(22.6)
Exchange differences	6.4	(2.6)
Net defined benefit liability (asset)	612.0	541.3
Funded \| Plan assets
Disclosure of net defined benefit liability (asset) [line items]
Net defined benefit liability (asset)	(462.7)	(429.8)
Interest cost/Interest income	(15.2)	(14.7)
Actuarial loss (gain) arising from:
Return on plan assets, excluding amounts included in interest income	6.7	19.4
Employee contributions	6.2	5.9
Employer contributions	21.2	18.9
Pension benefits paid	19.0	22.6
Administrative costs	(1.4)	(1.2)
Exchange differences	5.6	(2.2)
Net defined benefit liability (asset)	(497.2)	(462.7)
Funded \| Canadian
Disclosure of net defined benefit liability (asset) [line items]
Current service cost	22.3	22.0
Interest cost/Interest income	(14.1)	(13.6)
Funded \| Canadian \| Present value of defined benefit obligation
Disclosure of net defined benefit liability (asset) [line items]
Net defined benefit liability (asset)	487.4	468.9
Current service cost	22.3	22.0
Interest cost/Interest income	16.7	16.5
Actuarial loss (gain) arising from:
Experience adjustments	0.3	1.4
Economic assumptions	27.1	13.5
Demographic assumptions	4.8	(19.1)
Employee contributions	6.0	5.7
Pension benefits paid	(17.8)	(21.5)
Exchange differences	0.0	0.0
Net defined benefit liability (asset)	546.8	487.4
Funded \| Canadian \| Plan assets
Disclosure of net defined benefit liability (asset) [line items]
Net defined benefit liability (asset)	(415.9)	(382.9)
Interest cost/Interest income	(14.1)	(13.6)
Actuarial loss (gain) arising from:
Return on plan assets, excluding amounts included in interest income	3.8	19.1
Employee contributions	6.0	5.7
Employer contributions	20.1	17.2
Pension benefits paid	17.8	21.5
Administrative costs	(1.2)	(1.1)
Exchange differences	0.0	0.0
Net defined benefit liability (asset)	(440.9)	(415.9)
Funded \| Foreign
Disclosure of net defined benefit liability (asset) [line items]
Current service cost	1.5	1.2
Interest cost/Interest income	(1.1)
Funded \| Foreign \| Present value of defined benefit obligation
Disclosure of net defined benefit liability (asset) [line items]
Net defined benefit liability (asset)	53.9	52.3
Current service cost	1.5	1.2
Interest cost/Interest income	1.2	1.3
Actuarial loss (gain) arising from:
Experience adjustments	0.2	0.3
Economic assumptions	3.0	1.8
Demographic assumptions	0.0	0.5
Employee contributions	0.2	0.2
Pension benefits paid	(1.2)	(1.1)
Exchange differences	6.4	(2.6)
Net defined benefit liability (asset)	65.2	53.9
Funded \| Foreign \| Plan assets
Disclosure of net defined benefit liability (asset) [line items]
Net defined benefit liability (asset)	(46.8)	(46.9)
Interest cost/Interest income	(1.1)	(1.1)
Actuarial loss (gain) arising from:
Return on plan assets, excluding amounts included in interest income	2.9	0.3
Employee contributions	0.2	0.2
Employer contributions	1.1	1.7
Pension benefits paid	1.2	1.1
Administrative costs	(0.2)	(0.1)
Exchange differences	5.6	(2.2)
Net defined benefit liability (asset)	(56.3)	(46.8)
Unfunded
Disclosure of net defined benefit liability (asset) [line items]
Current service cost	2.3	2.2
Interest cost/Interest income	2.2	2.1
Unfunded \| Present value of defined benefit obligation
Disclosure of net defined benefit liability (asset) [line items]
Net defined benefit liability (asset)	79.1	76.6
Current service cost	2.3	2.2
Interest cost/Interest income	2.2	2.1
Past service cost, settlements and curtailments	0.0	0.1
Actuarial loss (gain) arising from:
Experience adjustments	(0.4)	(1.1)
Economic assumptions	(3.5)	(1.3)
Demographic assumptions	(0.4)	0.0
Pension benefits paid	3.6	3.6
Exchange differences	(1.5)	0.7
Net defined benefit liability (asset)	85.8	79.1
Unfunded \| Canadian
Disclosure of net defined benefit liability (asset) [line items]
Current service cost	2.3	2.2
Interest cost/Interest income	2.0	1.9
Unfunded \| Canadian \| Present value of defined benefit obligation
Disclosure of net defined benefit liability (asset) [line items]
Net defined benefit liability (asset)	66.2	62.9
Current service cost	2.3	2.2
Interest cost/Interest income	2.0	1.9
Past service cost, settlements and curtailments	0.0	0.0
Actuarial loss (gain) arising from:
Experience adjustments	(0.3)	(1.1)
Economic assumptions	(3.8)	(1.0)
Demographic assumptions	(0.4)	0.0
Pension benefits paid	2.8	2.9
Exchange differences	0.0	0.0
Net defined benefit liability (asset)	72.2	66.2
Unfunded \| Foreign
Disclosure of net defined benefit liability (asset) [line items]
Current service cost	0.0	0.0
Interest cost/Interest income	0.2	0.2
Unfunded \| Foreign \| Present value of defined benefit obligation
Disclosure of net defined benefit liability (asset) [line items]
Net defined benefit liability (asset)	12.9	13.7
Current service cost	0.0	0.0
Interest cost/Interest income	0.2	0.2
Past service cost, settlements and curtailments	0.0	0.1
Actuarial loss (gain) arising from:
Experience adjustments	(0.1)	0.0
Economic assumptions	0.3	(0.3)
Demographic assumptions	0.0	0.0
Pension benefits paid	0.8	0.7
Exchange differences	(1.5)	0.7
Net defined benefit liability (asset)	$ 13.6	$ 12.9

Administrative Expenses, Net Defined Benefit Liability (Asset)

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- Definition

The decrease (increase) in a net defined benefit liability (asset) resulting from actuarial gains (losses) arising from changes in demographic assumptions that result in remeasurements of the net defined benefit liability (asset). Demographic assumptions deal with matters such as: (a) mortality; (b) rates of employee turnover, disability and early retirement; (c) the proportion of plan members with dependants who will be eligible for benefits; (d) the proportion of plan members who will select each form of payment option available under the plan terms; and (e) claim rates under medical plans. [Refer: Net defined benefit liability (asset)]

+ References

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- Definition

The decrease (increase) in a net defined benefit liability (asset) resulting from actuarial gains (losses) arising from changes in financial assumptions that result in remeasurements of the net defined benefit liability (asset). Financial assumptions deal with items such as: (a) the discount rate; (b) benefit levels, excluding any cost of the benefits to be met by employees, and future salary; (c) in the case of medical benefits, future medical costs, including claim handling costs (ie the costs that will be incurred in processing and resolving claims, including legal and adjuster's fees); and (d) taxes payable by the plan on contributions relating to service before the reporting date or on benefits resulting from that service. [Refer: Net defined benefit liability (asset)]

+ References

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The decrease (increase) in a net defined benefit liability (asset) resulting from actuarial gains (losses) arising from experience adjustments that result in remeasurements of the net defined benefit liability (asset). Experience adjustments deal with the effects of differences between the previous actuarial assumptions and what has actually occurred. [Refer: Net defined benefit liability (asset)]

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
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- Definition

The decrease (increase) in net defined benefit liability (asset) resulting from contributions to a defined benefit plan by the employer. [Refer: Net defined benefit liability (asset); Defined benefit plans [member]]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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- Definition

The decrease (increase) in the net defined benefit liability (asset) resulting from contributions to a defined benefit plan by plan participants. [Refer: Net defined benefit liability (asset); Defined benefit plans [member]]

+ References

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- Definition

The increase (decrease) in the net defined benefit liability (asset) resulting from employee service in the current period. [Refer: Net defined benefit liability (asset)]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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Line items represent concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes of the table.

+ References

No definition available.

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- References

No definition available.

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- Definition

The increase (decrease) in the net defined benefit liability (asset) resulting from changes in foreign exchange rates. [Refer: Net defined benefit liability (asset)]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
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- Definition

The increase (decrease) in the net defined benefit liability (asset) resulting from the passage of time. [Refer: Interest expense; Net defined benefit liability (asset)]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 19
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- Definition

The amount of deficit or surplus in a defined benefit plan, adjusted for any effect of limiting a net defined benefit asset to the asset ceiling. [Refer: Defined benefit plans [member]]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
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- Definition

The increase (decrease) in the net defined benefit liability (asset) resulting from past service cost and gains (losses) arising from settlements. [Refer: Gains (losses) arising from settlements, net defined benefit liability (asset); Net defined benefit liability (asset)]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 19
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- Definition

The decrease (increase) in the net defined benefit liability (asset) resulting from payments from the plan. [Refer: Net defined benefit liability (asset)]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
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- Definition

The decrease (increase) in the net defined benefit liability (asset) resulting from the return on plan assets. The return on plans assets is interest, dividends and other revenue derived from the plan assets, together with realised and unrealised gains or losses on the plan assets, less any costs of managing plan assets and less any tax payable by the plan itself, other than tax included in the actuarial assumptions used to measure the present value of the defined benefit obligation. [Refer: Plan assets [member]; Net defined benefit liability (asset); Actuarial assumptions [member]]

+ References

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