EMPLOYEE BENEFITS (Changes in Benefit Obligations, Fair Value of Plan Assets and Unfunded Status) (Details) $ in Millions, $ in Millions			12 Months Ended
	Oct. 31, 2017 CAD ($)	Oct. 31, 2017 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of period			$ 93.5
Fair value of pension plan assets at end of period			93.4	$ 93.5
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Fair value of plan assets			93.5	93.5	$ 93.4	$ 93.5
Allstream [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets transferred resulting from Allstream acquisition	$ 117.9	$ 91.6
Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation					106.6	103.9
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Projected benefit obligation at beginning of period			113.5	6.7
Service cost			2.8	2.7
Interest cost			3.7	2.5
Actuarial loss			3.8	9.3
Participant contributions			0.6	0.6
Benefits paid from plan assets			(1.5)	(3.3)
Acquisition transfer				97.6
Curtailments			(2.3)
Settlements			(8.3)	(2.5)
Foreign currency exchange rate changes			0.4	(0.1)
Projected benefit obligation at end of period			112.7	113.5
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of period			93.5	3.7
Acquisition transfer				89.8
Return on plan assets			5.7	2.4
Employer contributions			3.1	2.9
Participant contributions			0.6	0.6
Benefits paid from plan assets			(1.5)	(3.3)
Settlements			(8.3)	(2.6)
Foreign currency exchange rate changes			0.3
Fair value of pension plan assets at end of period			93.4	93.5
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Projected benefit obligation			113.5	6.7	112.7	113.5
Fair value of plan assets			93.5	3.7	93.4	93.5
Unfunded status					(19.3)	(20.0)
Non-current portion of unfunded status					19.3	20.0
OPEB Plans
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation					10.6	10.0
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Projected benefit obligation at beginning of period			10.0	9.9
Service cost			0.1	0.1
Interest cost			0.3	0.3
Actuarial loss			0.8	0.4
Benefits paid from plan assets			(0.6)	(0.6)
Foreign currency exchange rate changes				(0.1)
Projected benefit obligation at end of period			10.6	10.0
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Projected benefit obligation			$ 10.0	$ 9.9	$ 10.6	$ 10.0