SEGMENT REPORTING (Summary of Financial Information by Segments) (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018	Jun. 30, 2018
Segment Reporting Information [Line Items]
Revenue from external customers	$ 647.2	$ 649.0	$ 1,927.4	$ 1,945.2
Segment Adjusted EBITDA	321.3	319.4	961.9	964.7
Total assets	9,263.8		9,263.8		$ 9,209.9
Capital expenditures	206.4	195.1	591.1	581.9
Reportable Segments [Member] \| Fiber Solutions [Member]
Segment Reporting Information [Line Items]
Revenue from external customers	237.6	213.3	687.2	615.1
Segment Adjusted EBITDA	187.5	170.9	549.0	498.3
Total assets	5,115.7		5,115.7		4,937.3
Capital expenditures	125.0	102.7	330.5	324.5
Reportable Segments [Member] \| Transport [Member]
Segment Reporting Information [Line Items]
Revenue from external customers	170.4	166.5	508.6	500.5
Segment Adjusted EBITDA	56.8	55.0	171.9	172.4
Total assets	1,881.1		1,881.1		1,870.9
Capital expenditures	48.4	49.0	136.9	134.5
Reportable Segments [Member] \| Enterprise Networks [Member]
Segment Reporting Information [Line Items]
Revenue from external customers	81.2	86.1	245.4	265.4
Segment Adjusted EBITDA	33.4	36.2	100.8	110.7
Total assets	757.4		757.4		754.6
Capital expenditures	9.8	15.7	39.7	36.2
Reportable Segments [Member] \| zColo [Member]
Segment Reporting Information [Line Items]
Revenue from external customers	57.4	59.6	173.9	177.9
Segment Adjusted EBITDA	27.5	28.7	83.4	88.3
Total assets	1,062.4		1,062.4		1,032.4
Capital expenditures	18.5	25.5	70.2	78.1
Reportable Segments [Member] \| Allstream [Member]
Segment Reporting Information [Line Items]
Revenue from external customers	92.0	117.7	293.7	368.9
Segment Adjusted EBITDA	14.4	26.8	52.8	91.2
Total assets	390.3		390.3		465.5
Capital expenditures	4.7	2.2	13.8	8.6
Reportable Segments [Member] \| Other [Member]
Segment Reporting Information [Line Items]
Revenue from external customers	8.6	5.8	18.6	17.4
Segment Adjusted EBITDA	1.9	1.7	4.2	3.9
Total assets	32.4		32.4		33.4
Corp/Eliminations [Member]
Segment Reporting Information [Line Items]
Segment Adjusted EBITDA	(0.2)	$ 0.1	(0.2)	$ (0.1)
Total assets	$ 24.5		$ 24.5		$ 115.8

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