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) CAD in Millions, $ in Millions			3 Months Ended	6 Months Ended
	Oct. 31, 2017 CAD	Oct. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Assets transferred resulting from Allstream acqusition	CAD 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			$ 97.2	97.2
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Fair value of plan assets			97.2	97.2	$ 97.2
Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation					99.2
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Projected benefit obligation at beginning of year				6.7
Service cost			0.7	1.5
Interest cost			0.6	0.7
Actuarial loss (gain)				(1.3)
Participant contributions				0.3
Benefits paid from plan assets				(3.6)
Acquisition transfer				102.1
Foreign currency exchange rate changes				0.1
Foreign currency exchange rate changes				0.2
Projected benefit obligation at end of year			106.6	106.6
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				1.2
Employer contributions				1.6
Participant contributions				0.3
Benefits paid from plan assets				(3.6)
Acquisition transfer				93.9
Foreign currency exchange rate changes				0.1
Fair value of plan assets at end of year			97.2	97.2
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Projected benefit obligation			106.6	6.7	106.6
Fair value of plan assets			97.2	3.7	97.2
Unfunded status					(9.4)
Non-current portion of unfunded status					9.4
Post-Retirement Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation					10.1
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	0.2
Benefits paid from plan assets				(0.3)
Foreign currency exchange rate changes				0.2
Projected benefit obligation at end of year			10.1	10.1
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Projected benefit obligation			$ 10.1	$ 9.9	$ 10.1