GOODWILL (Schedule of Goodwill) (Details) - USD ($) $ in Millions	12 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Goodwill [Line Items]
Goodwill, beginning balance	$ 1,719.1	$ 1,840.2
Adjustments to Prior Acquisitions	(6.7)	(165.5)
Fiscal Acquisitions		47.4
Foreign Currency Translation and Other	(5.8)	(3.0)
Goodwill, ending balance	1,706.6	1,719.1
Fiber Solutions [Member]
Goodwill [Line Items]
Goodwill, beginning balance	756.4	633.9
Adjustments to Prior Acquisitions	(5.2)	88.6
Fiscal Acquisitions		33.1
Reallocation among reporting units	253.6
Foreign Currency Translation and Other	(3.9)	0.8
Goodwill, ending balance	1,000.9	756.4
Layer 2/3 [Member]
Goodwill [Line Items]
Reallocation among reporting units	201.6
Foreign Currency Translation and Other	(0.8)
Goodwill, ending balance	200.8
Transport [Member]
Goodwill [Line Items]
Reallocation among reporting units	176.8
Foreign Currency Translation and Other	(0.6)
Goodwill, ending balance	176.2
Waves [Member]
Goodwill [Line Items]
Goodwill, beginning balance	194.8	247.4
Adjustments to Prior Acquisitions	(1.3)	(56.3)
Fiscal Acquisitions		13.6
Reallocation among reporting units	(193.5)	(10.5)
Foreign Currency Translation and Other		0.6
Goodwill, ending balance		194.8
Sonet [Member]
Goodwill [Line Items]
Goodwill, beginning balance	87.6	52.0
Adjustments to Prior Acquisitions		35.6
Reallocation among reporting units	(87.6)
Goodwill, ending balance		87.6
Ethernet [Member]
Goodwill [Line Items]
Goodwill, beginning balance	104.2	359.5
Adjustments to Prior Acquisitions	(0.2)	(249.8)
Fiscal Acquisitions		0.7
Reallocation among reporting units	(104.0)	(6.3)
Foreign Currency Translation and Other		0.1
Goodwill, ending balance		104.2
Live Video [Member]
Goodwill [Line Items]
Goodwill, beginning balance	3.3
Reallocation among reporting units	(3.3)	3.3
Goodwill, ending balance		3.3
WANs [Member]
Goodwill [Line Items]
Goodwill, beginning balance	179.3	89.5
Adjustments to Prior Acquisitions		89.7
Reallocation among reporting units	(179.3)
Foreign Currency Translation and Other		0.1
Goodwill, ending balance		179.3
zColo [Member]
Goodwill [Line Items]
Goodwill, beginning balance	260.1	256.3
Adjustments to Prior Acquisitions		3.2
Foreign Currency Translation and Other	(0.5)	0.6
Goodwill, ending balance	259.6	260.1
Cloud [Member]
Goodwill [Line Items]
Goodwill, beginning balance	65.3	69.5
Adjustments to Prior Acquisitions		(4.2)
Reallocation among reporting units	(50.6)
Goodwill, ending balance	14.7	65.3
Cloudlink [Member]
Goodwill [Line Items]
Goodwill, beginning balance	13.5
Reallocation among reporting units	(13.5)	13.5
Goodwill, ending balance		13.5
Allstream [Member]
Goodwill [Line Items]
Goodwill, beginning balance	39.0	116.5
Adjustments to Prior Acquisitions		(72.3)
Foreign Currency Translation and Other		(5.2)
Goodwill, ending balance	39.0	39.0
Other [Member]
Goodwill [Line Items]
Goodwill, beginning balance	15.6	15.6
Reallocation among reporting units	(0.2)
Goodwill, ending balance	$ 15.4	$ 15.6