GOODWILL (Schedule of Goodwill) (Details) $ in Millions	6 Months Ended
	Dec. 31, 2019 USD ($)
Goodwill [Line Items]
Goodwill, beginning balance	$ 1,706.6
Foreign Currency Translation and Other	4.5
Goodwill, ending balance	1,711.1
Fiber Solutions [Member]
Goodwill [Line Items]
Goodwill, beginning balance	1,000.9
Foreign Currency Translation and Other	3.6
Goodwill, ending balance	1,004.5
Layer 2/3 [Member]
Goodwill [Line Items]
Goodwill, beginning balance	200.8
Foreign Currency Translation and Other	0.7
Goodwill, ending balance	201.5
Transport [Member]
Goodwill [Line Items]
Goodwill, beginning balance	176.2
Foreign Currency Translation and Other	0.6
Goodwill, ending balance	176.8
zColo [Member]
Goodwill [Line Items]
Goodwill, beginning balance	259.6
Foreign Currency Translation and Other	(0.4)
Goodwill, ending balance	259.2
Cloud [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