ACQUISITIONS AND DISPOSITIONS (Narrative) (Details) € in Millions, $ in Millions, $ in Millions															12 Months Ended
	Apr. 17, 2018 USD ($)	Feb. 28, 2018 USD ($)	Jan. 18, 2018 CAD ($)	Jan. 18, 2018 USD ($)	Oct. 31, 2017 CAD ($)	Oct. 31, 2017 USD ($)	May 01, 2017 USD ($)	Mar. 01, 2017 USD ($)	Oct. 03, 2016 USD ($) payment	Apr. 01, 2016 USD ($) item	Jan. 15, 2016 CAD ($)	Jan. 15, 2016 USD ($)	Dec. 31, 2015 EUR (€)	Dec. 31, 2015 USD ($)	Jun. 30, 2018 CAD ($) item	Jun. 30, 2018 USD ($) item	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)
Business Acquisition [Line Items]
Number of business combinations completed | item															45	45
Increase (decrease) in intangible assets																$ 103.9
Increase in property plant and equipment																160.8
Increase in depreciation and amortization																18.6	$ 6.2
Acquisition related costs																18.6	20.5	$ 21.5
Net consideration paid																176.9	1,434.8	$ 437.5
Neutral path communications and near north partners
Business Acquisition [Line Items]
Business acquisition purchase price	$ 33.3
Deferred tax assets, net	$ 1.5
Purchase price, held in escrow																4.0
Spread Networks
Business Acquisition [Line Items]
Business acquisition purchase price		$ 130.5
Deferred tax assets, net		$ 7.1
Purchase price, held in escrow																0.6
Optic Zoo Networks
Business Acquisition [Line Items]
Business acquisition purchase price			$ 30.9	$ 24.8
Purchase price, held in escrow															$ 3.1	2.4
KIO Networks US Data Centers
Business Acquisition [Line Items]
Business acquisition purchase price							$ 11.9
Deferred tax assets, net							$ 2.2
Electric Lightwave
Business Acquisition [Line Items]
Business acquisition purchase price								$ 1,426.6
Deferred tax assets, net																	46.7
Santa Clara Data Center
Business Acquisition [Line Items]
Business acquisition purchase price									$ 11.3
Remaining cash consideration to be paid																3.8
Number of quarterly payments | payment									10
Amount of quarterly payments									$ 1.3							$ 5.0	3.7
Clearview
Business Acquisition [Line Items]
Business acquisition purchase price										$ 18.3
Deferred tax assets, net										$ 0.2
Number of data centers | item										2
Business acquisition, percentage of voting interests acquired										100.00%
Allstream
Business Acquisition [Line Items]
Business acquisition purchase price											$ 422.9	$ 297.6
Deferred tax assets, net												$ 3.8
Business acquisition, percentage of voting interests acquired												100.00%
Working capital and other liabilities assumed in the acquisition												$ 29.6
Assets transferred resulting from Allstream acquisition					$ 117.9	$ 91.6
Viatel
Business Acquisition [Line Items]
Business acquisition purchase price														$ 101.2
Net consideration paid													€ 92.9	$ 101.2
Business acquisition, percentage of voting interests acquired													100.00%	100.00%
Refund of escrow reflected as a cash inflow from investing activities																	$ 1.5
Dallas Data Center
Business Acquisition [Line Items]
Business acquisition purchase price														$ 16.6