EMPLOYEE BENEFITS (Changes in Benefit Obligations, Fair Value of Plan Assets and Underfunded Status for Company's Defined Benefit Pension and Postretirement Benefit Plans) (Details) $ in Millions, $ in Millions			3 Months Ended	9 Months Ended
	Oct. 31, 2017 CAD ($)	Oct. 31, 2017 USD ($)	Mar. 31, 2018 USD ($)	Mar. 31, 2018 USD ($)	Mar. 31, 2018 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Assets transferred resulting from Allstream acquisition	$ 117.9	$ 91.6
Amounts reclassified from AOCI				$ 0.0
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at end of year			$ 93.4	93.4
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Fair value of plan assets			93.4	93.4	$ 93.4
Pension plans
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation					101.5
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Projected benefit obligation at beginning of year				6.7
Service cost			0.7	2.1
Interest cost			0.9	1.6
Actuarial loss (gain)				4.0
Participant contributions				0.5
Benefits paid from plan assets				(5.1)
Acquisition transfer				99.4
Projected benefit obligation at end of year			109.2	109.2
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year				3.7
Return on plan assets				0.4
Employer contributions				2.5
Participant contributions				0.5
Benefits paid from plan assets				(5.1)
Acquisition transfer				91.4
Fair value of plan assets at end of year			93.4	93.4
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Projected benefit obligation			109.2	6.7	109.2
Fair value of plan assets			93.4	3.7	93.4
Unfunded status					(15.8)
Non-current portion of unfunded status					15.8
Post-retirement benefit plans
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Projected benefit obligation at beginning of year				9.9
Service cost				0.1
Interest cost			0.1	0.3
Actuarial loss (gain)				0.4
Benefits paid from plan assets			(9.8)	(0.4)
Foreign currency exchange rate changes				0.1
Projected benefit obligation at end of year			10.4	10.4
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Projected benefit obligation			$ 10.4	$ 9.9	$ 10.4