GOODWILL (Schedule of Goodwill) (Details) $ in Millions	3 Months Ended
	Sep. 30, 2018 USD ($)
Goodwill [Line Items]
Goodwill, beginning Balance	$ 1,719.1
Adjustments to Prior Acquisitions	(7.2)
Foreign Currency Translation and Other	1.7
Goodwill, ending balance	1,710.2
Fiber Solutions [Member]
Goodwill [Line Items]
Goodwill, beginning Balance	756.4
Adjustments to Prior Acquisitions	(6.1)
Foreign Currency Translation and Other	0.9
Goodwill, ending balance	749.4
Waves [Member]
Goodwill [Line Items]
Goodwill, beginning Balance	194.8
Adjustments to Prior Acquisitions	(0.9)
Foreign Currency Translation and Other	0.8
Goodwill, ending balance	193.1
Sonet [Member]
Goodwill [Line Items]
Goodwill, beginning Balance	87.6
Goodwill, ending balance	87.6
Ethernet [Member]
Goodwill [Line Items]
Goodwill, beginning Balance	104.2
Adjustments to Prior Acquisitions	(0.2)
Goodwill, ending balance	104.0
Live Video [Member]
Goodwill [Line Items]
Goodwill, beginning Balance	3.3
Goodwill, ending balance	3.3
WANs [Member]
Goodwill [Line Items]
Goodwill, beginning Balance	179.3
Foreign Currency Translation and Other	(0.1)
Goodwill, ending balance	179.4
zColo [Member]
Goodwill [Line Items]
Goodwill, beginning Balance	260.1
Foreign Currency Translation and Other	0.1
Goodwill, ending balance	260.0
Cloud [Member]
Goodwill [Line Items]
Goodwill, beginning Balance	65.3
Goodwill, ending balance	65.3
Cloudlink [Member]
Goodwill [Line Items]
Goodwill, beginning Balance	13.5
Goodwill, ending balance	13.5
Allstream [Member]
Goodwill [Line Items]
Goodwill, beginning Balance	39.0
Goodwill, ending balance	39.0
Other [Member]
Goodwill [Line Items]
Goodwill, beginning Balance	15.6
Goodwill, ending balance	$ 15.6