GOODWILL (Schedule of Goodwill) (Details) $ in Millions	3 Months Ended
	Sep. 30, 2019 USD ($)
Goodwill [Line Items]
Goodwill, beginning balance	$ 1,706.6
Foreign Currency Translation and Other	5.5
Goodwill, ending balance	1,701.1
Fiber Solutions [Member]
Goodwill [Line Items]
Goodwill, beginning balance	1,000.9
Foreign Currency Translation and Other	3.2
Goodwill, ending balance	997.7
Layer 2/3 [Member]
Goodwill [Line Items]
Goodwill, beginning balance	200.8
Foreign Currency Translation and Other	(0.7)
Goodwill, ending balance	200.1
Transport [Member]
Goodwill [Line Items]
Goodwill, beginning balance	176.2
Foreign Currency Translation and Other	(0.6)
Goodwill, ending balance	175.6
zColo [Member]
Goodwill [Line Items]
Goodwill, beginning balance	259.6
Foreign Currency Translation and Other	1.0
Goodwill, ending balance	258.6
Cloudlink [Member]
Goodwill [Line Items]
Goodwill, beginning balance	14.7
Goodwill, ending balance	14.7
Allstream [Member]
Goodwill [Line Items]
Goodwill, beginning balance	39.0
Goodwill, ending balance	39.0
Other [Member]
Goodwill [Line Items]
Goodwill, beginning balance	15.4
Goodwill, ending balance	$ 15.4