REVENUE RECOGNITION (Remaining Performance Obligation) (Details) - USD ($) $ in Millions	3 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
REVENUE RECOGNITION
Revenue, remaining performance obligation	$ 1,390.2
Fiber Solutions [Member]
REVENUE RECOGNITION
Revenue, remaining performance obligation	10.2
Transport [Member]
REVENUE RECOGNITION
Revenue, remaining performance obligation	736.3
Enterprise Networks [Member]
REVENUE RECOGNITION
Revenue, remaining performance obligation	392.8
zColo [Member]
REVENUE RECOGNITION
Revenue, remaining performance obligation	68.1
Allstream [Member]
REVENUE RECOGNITION
Revenue, remaining performance obligation	182.8
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-10-01
REVENUE RECOGNITION
Revenue, remaining performance obligation	$ 643.6
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue recognition satisfaction period	9 months
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-10-01 | Fiber Solutions [Member]
REVENUE RECOGNITION
Revenue, remaining performance obligation	$ 8.8
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-10-01 | Transport [Member]
REVENUE RECOGNITION
Revenue, remaining performance obligation	313.5
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-10-01 | Enterprise Networks [Member]
REVENUE RECOGNITION
Revenue, remaining performance obligation	180.3
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-10-01 | zColo [Member]
REVENUE RECOGNITION
Revenue, remaining performance obligation	25.6
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-10-01 | Allstream [Member]
REVENUE RECOGNITION
Revenue, remaining performance obligation	115.4
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-07-01
REVENUE RECOGNITION
Revenue, remaining performance obligation	$ 440.4
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue recognition satisfaction period	12 months
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-07-01 | Fiber Solutions [Member]
REVENUE RECOGNITION
Revenue, remaining performance obligation	$ 1.1
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-07-01 | Transport [Member]
REVENUE RECOGNITION
Revenue, remaining performance obligation	236.3
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-07-01 | Enterprise Networks [Member]
REVENUE RECOGNITION
Revenue, remaining performance obligation	133.1
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-07-01 | zColo [Member]
REVENUE RECOGNITION
Revenue, remaining performance obligation	20.3
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-07-01 | Allstream [Member]
REVENUE RECOGNITION
Revenue, remaining performance obligation	49.6
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2020-07-01
REVENUE RECOGNITION
Revenue, remaining performance obligation	$ 196.7
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue recognition satisfaction period	12 months
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2020-07-01 | Fiber Solutions [Member]
REVENUE RECOGNITION
Revenue, remaining performance obligation	$ 0.3
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2020-07-01 | Transport [Member]
REVENUE RECOGNITION
Revenue, remaining performance obligation	112.6
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2020-07-01 | Enterprise Networks [Member]
REVENUE RECOGNITION
Revenue, remaining performance obligation	58.8
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2020-07-01 | zColo [Member]
REVENUE RECOGNITION
Revenue, remaining performance obligation	10.4
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2020-07-01 | Allstream [Member]
REVENUE RECOGNITION
Revenue, remaining performance obligation	14.6
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-07-01
REVENUE RECOGNITION
Revenue, remaining performance obligation	$ 59.3
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue recognition satisfaction period	12 months
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-07-01 | Transport [Member]
REVENUE RECOGNITION
Revenue, remaining performance obligation	$ 36.8
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-07-01 | Enterprise Networks [Member]
REVENUE RECOGNITION
Revenue, remaining performance obligation	14.9
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-07-01 | zColo [Member]
REVENUE RECOGNITION
Revenue, remaining performance obligation	5.1
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-07-01 | Allstream [Member]
REVENUE RECOGNITION
Revenue, remaining performance obligation	2.5
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2022-07-01
REVENUE RECOGNITION
Revenue, remaining performance obligation	$ 24.9
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue recognition satisfaction period	12 months
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2022-07-01 | Transport [Member]
REVENUE RECOGNITION
Revenue, remaining performance obligation	$ 16.5
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2022-07-01 | Enterprise Networks [Member]
REVENUE RECOGNITION
Revenue, remaining performance obligation	5.0
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2022-07-01 | zColo [Member]
REVENUE RECOGNITION
Revenue, remaining performance obligation	2.8
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2022-07-01 | Allstream [Member]
REVENUE RECOGNITION
Revenue, remaining performance obligation	0.6
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2023-07-01
REVENUE RECOGNITION
Revenue, remaining performance obligation	25.3
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue recognition satisfaction period
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2023-07-01 | Transport [Member]
REVENUE RECOGNITION
Revenue, remaining performance obligation	20.6
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2023-07-01 | Enterprise Networks [Member]
REVENUE RECOGNITION
Revenue, remaining performance obligation	0.7
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2023-07-01 | zColo [Member]
REVENUE RECOGNITION
Revenue, remaining performance obligation	3.9
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2023-07-01 | Allstream [Member]
REVENUE RECOGNITION
Revenue, remaining performance obligation	$ 0.1