Goodwill (Schedule Of Goodwill) (Details) - USD ($) $ in Millions	12 Months Ended
	Jun. 30, 2017	Jun. 30, 2016
Goodwill [Line Items]
Goodwill, beginning Balance	$ 1,214.5	$ 1,224.4
Adjustments to Fiscal Acquisitions	(5.3)	(5.5)
Fiscal Acquisitions	635.3	16.9
Foreign Currency Translation and Other	(4.3)	(21.3)
Goodwill, ending balance	1,840.2	1,214.5
Dark Fiber [Member]
Goodwill [Line Items]
Goodwill, beginning Balance	295.1	299.1
Reallocation among reporting units	(295.1)
Fiscal Acquisitions		10.2
Foreign Currency Translation and Other		(14.2)
Goodwill, ending balance		295.1
Fiber Solutions [Member]
Goodwill [Line Items]
Reallocation among reporting units	544.8
Adjustments to Fiscal Acquisitions	1.7
Fiscal Acquisitions	92.9
Foreign Currency Translation and Other	(5.5)
Goodwill, ending balance	633.9
Waves [Member]
Goodwill [Line Items]
Goodwill, beginning Balance	258.3	265.6
Reallocation among reporting units	(104.0)
Adjustments to Fiscal Acquisitions	(2.7)
Fiscal Acquisitions	96.5
Foreign Currency Translation and Other	(0.7)	(7.3)
Goodwill, ending balance	247.4	258.3
Sonet [Member]
Goodwill [Line Items]
Goodwill, beginning Balance	51.3	50.3
Reallocation among reporting units	0.7
Fiscal Acquisitions		1.0
Goodwill, ending balance	52.0	51.3
Ethernet [Member]
Goodwill [Line Items]
Goodwill, beginning Balance	104.3	104.2
Reallocation among reporting units	(59.8)
Adjustments to Fiscal Acquisitions	(0.5)
Fiscal Acquisitions	315.3
Foreign Currency Translation and Other	0.2	0.1
Goodwill, ending balance	359.5	104.3
IP [Member]
Goodwill [Line Items]
Goodwill, beginning Balance	87.5	86.3
Reallocation among reporting units	(87.5)
Fiscal Acquisitions		1.4
Foreign Currency Translation and Other		(0.2)
Goodwill, ending balance		87.5
MIG [Member]
Goodwill [Line Items]
Goodwill, beginning Balance	73.6	73.4
Reallocation among reporting units	(73.6)
Adjustments to Fiscal Acquisitions		0.2
Goodwill, ending balance		73.6
EPIC [Member]
Goodwill [Line Items]
Reallocation among reporting units	89.5
Goodwill, ending balance	89.5
zColo [Member]
Goodwill [Line Items]
Goodwill, beginning Balance	268.8	273.2
Reallocation among reporting units	(15.0)
Adjustments to Fiscal Acquisitions	(3.7)	(5.7)
Fiscal Acquisitions	4.7	1.3
Foreign Currency Translation and Other	1.5
Goodwill, ending balance	256.3	268.8
Cloud [Member]
Goodwill [Line Items]
Goodwill, beginning Balance	60.0	57.0
Adjustments to Fiscal Acquisitions	(0.1)
Fiscal Acquisitions	9.4	3.0
Foreign Currency Translation and Other	0.2
Goodwill, ending balance	69.5	60.0
Allstream
Goodwill [Line Items]
Fiscal Acquisitions	116.5
Goodwill, ending balance	116.5
Other [Member]
Goodwill [Line Items]
Goodwill, beginning Balance	15.6	15.3
Foreign Currency Translation and Other		0.3
Goodwill, ending balance	$ 15.6	$ 15.6