GOODWILL (Schedule of Goodwill) (Details) - USD ($) $ in Millions	12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Goodwill [Line Items]
Goodwill, beginning Balance	$ 1,840.2	$ 1,214.5
Adjustments to Prior Acquisitions	(165.5)	(5.3)
Fiscal Acquisitions	47.4	635.3
Foreign Currency Translation and Other	3.0	(4.3)
Goodwill, ending balance	1,719.1	1,840.2
Dark Fiber [Member]
Goodwill [Line Items]
Goodwill, beginning Balance		295.1
Reallocation among reporting units		(295.1)
Fiber Solutions [Member]
Goodwill [Line Items]
Goodwill, beginning Balance	633.9
Reallocation among reporting units		544.8
Adjustments to Prior Acquisitions	88.6	1.7
Fiscal Acquisitions	33.1	92.9
Foreign Currency Translation and Other	(0.8)	(5.5)
Goodwill, ending balance	756.4	633.9
Waves [Member]
Goodwill [Line Items]
Goodwill, beginning Balance	247.4	258.3
Reallocation among reporting units	(10.5)	(104.0)
Adjustments to Prior Acquisitions	(56.3)	(2.7)
Fiscal Acquisitions	13.6	96.5
Foreign Currency Translation and Other	(0.6)	(0.7)
Goodwill, ending balance	194.8	247.4
Sonet [Member]
Goodwill [Line Items]
Goodwill, beginning Balance	52.0	51.3
Reallocation among reporting units		0.7
Adjustments to Prior Acquisitions	35.6
Goodwill, ending balance	87.6	52.0
Ethernet [Member]
Goodwill [Line Items]
Goodwill, beginning Balance	359.5	104.3
Reallocation among reporting units	(6.3)	(59.8)
Adjustments to Prior Acquisitions	(249.8)	(0.5)
Fiscal Acquisitions	0.7	315.3
Foreign Currency Translation and Other	(0.1)	0.2
Goodwill, ending balance	104.2	359.5
Live Video
Goodwill [Line Items]
Reallocation among reporting units	3.3
Goodwill, ending balance	3.3
WANs [Member]
Goodwill [Line Items]
Goodwill, beginning Balance	89.5
Reallocation among reporting units		89.5
Adjustments to Prior Acquisitions	89.7
Foreign Currency Translation and Other	(0.1)
Goodwill, ending balance	179.3	89.5
IP [Member]
Goodwill [Line Items]
Goodwill, beginning Balance		87.5
Reallocation among reporting units		(87.5)
MIG [Member]
Goodwill [Line Items]
Goodwill, beginning Balance		73.6
Reallocation among reporting units		(73.6)
zColo [Member]
Goodwill [Line Items]
Goodwill, beginning Balance	256.3	268.8
Reallocation among reporting units		(15.0)
Adjustments to Prior Acquisitions	3.2	(3.7)
Fiscal Acquisitions		4.7
Foreign Currency Translation and Other	(0.6)	1.5
Goodwill, ending balance	260.1	256.3
Cloud [Member]
Goodwill [Line Items]
Goodwill, beginning Balance	69.5	60.0
Adjustments to Prior Acquisitions	(4.2)	(0.1)
Fiscal Acquisitions		9.4
Foreign Currency Translation and Other		0.2
Goodwill, ending balance	65.3	69.5
Cloudlink
Goodwill [Line Items]
Reallocation among reporting units	13.5
Goodwill, ending balance	13.5
Allstream
Goodwill [Line Items]
Goodwill, beginning Balance	116.5
Adjustments to Prior Acquisitions	(72.3)
Fiscal Acquisitions		116.5
Foreign Currency Translation and Other	5.2
Goodwill, ending balance	39.0	116.5
Other [Member]
Goodwill [Line Items]
Goodwill, beginning Balance	15.6	15.6
Goodwill, ending balance	$ 15.6	$ 15.6