SEGMENT REPORTING (Summary of Financial Information by Segments) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2016
Segment Reporting Information [Line Items]
Revenue from external customers	$ 657.6	$ 649.4	$ 653.5	$ 643.5	$ 638.0	$ 550.2	$ 506.7	$ 504.9	$ 2,604.0	$ 2,199.8	$ 1,721.7
Segment Adjusted EBITDA									1,291.0	1,116.8	934.9
Total assets	9,216.8				8,739.4				9,216.8	8,739.4
Capital expenditures									789.9	835.5	704.1
Fiber Solutions [Member]
Segment Reporting Information [Line Items]
Revenue from external customers									842.0
Segment Adjusted EBITDA									673.2
Total assets	4,956.9								4,956.9
Capital expenditures									443.3
Reportable Segments [Member] | Fiber Solutions [Member]
Segment Reporting Information [Line Items]
Revenue from external customers										725.4	648.8
Segment Adjusted EBITDA										570.4	504.0
Total assets					4,504.6					4,504.6
Capital expenditures										499.1	421.4
Reportable Segments [Member] | Transport [Member]
Segment Reporting Information [Line Items]
Revenue from external customers									670.4	614.3	522.6
Segment Adjusted EBITDA									230.3	223.0	208.4
Total assets	1,864.6				2,018.3				1,864.6	2,018.3
Capital expenditures									183.1	189.6	160.6
Impact of pricing changes	(1.8)
Reportable Segments [Member] | Enterprise Networks [Member]
Segment Reporting Information [Line Items]
Revenue from external customers									350.4	310.2	228.4
Segment Adjusted EBITDA									148.2	127.9	100.6
Total assets	752.0				597.4				752.0	597.4
Capital expenditures									50.0	46.8	50.5
Impact of pricing changes	2.7
Reportable Segments [Member] | zColo [Member]
Segment Reporting Information [Line Items]
Revenue from external customers									238.2	214.0	195.4
Segment Adjusted EBITDA									120.3	107.1	99.5
Total assets	1,032.4				994.4				1,032.4	994.4
Capital expenditures									99.9	91.5	66.3
Impact of pricing changes	0.1
Reportable Segments [Member] | Allstream
Segment Reporting Information [Line Items]
Revenue from external customers									479.9	314.3	106.2
Segment Adjusted EBITDA									114.0	83.3	17.9
Total assets	461.8				406.2				461.8	406.2
Capital expenditures									13.6	8.5	5.3
Impact of pricing changes	(1.0)
Reportable Segments [Member] | Other [Member]
Segment Reporting Information [Line Items]
Revenue from external customers									23.1	21.6	20.3
Segment Adjusted EBITDA									5.0	5.1	$ 4.5
Total assets	33.3				33.2				33.3	33.2
Corp/Eliminations [Member]
Segment Reporting Information [Line Items]
Total assets	$ 115.8				$ 185.3				$ 115.8	$ 185.3