BASIS OF PRESENTATION AND SIGNIFICANT ACCOUNTING POLICIES (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								4 Months Ended	8 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	Feb. 28, 2018	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2016
Basis Of Presentation And Significant Accounting Policies [Line Items]
Weighted-average shares included to account for the dilutive effect of the Part A and Part B RSUs											1.2	2.9
Depreciation expense											$ (650.2)	$ (526.9)	$ (440.5)
Net income/(loss)											$ 101.9	$ 85.7	$ (76.2)
Basic and diluted earnings per share	$ 0.18	$ 0.09	$ 0.05	$ 0.09	$ 0.09	$ 0.11	$ 0.08	$ 0.06			$ 0.41	$ 0.35	$ (0.31)
Impairment of property, plant and equipment											$ 0.0	$ 0.0	$ 0.0
Impairment of goodwill and intangibles											0.0	0.0	0.0
Assets of parent company	$ 9,216.8				$ 8,739.4				$ 9,216.8		9,216.8	8,739.4
Liabilities of parent company	7,727.7				7,328.9				7,727.7		7,727.7	7,328.9
Excess tax benefit from stock-based compensation												15.6	$ 7.9
Cumulative effect adjustment resulting from adoption of ASU 2016-09											1.7
Parent Company [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Assets of parent company	0.0								0.0		0.0
Liabilities of parent company	0.0								0.0		0.0
Cash of parent company	$ 0.7				$ 0.7				$ 0.7		$ 0.7	$ 0.7
Allstream
Basis Of Presentation And Significant Accounting Policies [Line Items]
Percentage of entity acquired	100.00%								100.00%		100.00%
Customer Concentration Risk [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Major customer concentration											10.00%	10.00%
Restricted Stock Units (RSUs) [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Adjustment for dilutive effect of Part A and Part B shares											4.6	4.5
Property Plant And Equipment Depreciatiion Period
Basis Of Presentation And Significant Accounting Policies [Line Items]
Depreciation expense											$ 69.6
Net income/(loss)											$ 55.4
Basic and diluted earnings per share											$ 0.22
Fiber optic network | Property Plant And Equipment Depreciatiion Period
Basis Of Presentation And Significant Accounting Policies [Line Items]
Estimated useful life in years									33 years
Owned buildings
Basis Of Presentation And Significant Accounting Policies [Line Items]
Estimated useful life in years										39 years
Owned buildings | Property Plant And Equipment Depreciatiion Period
Basis Of Presentation And Significant Accounting Policies [Line Items]
Estimated useful life in years									45 years
Customer relationships [member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Customer relationship estimated life											15 years 1 month 6 days
Minimum | Parent Company [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Restricted net assets of parent company exceeds consolidated net assets	25.00%								25.00%		25.00%
Minimum | Fiber optic network
Basis Of Presentation And Significant Accounting Policies [Line Items]
Estimated useful life in years										15 years	33 years
Minimum | Customer relationships [member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Customer relationship estimated life											10 years
Maximum | Fiber optic network
Basis Of Presentation And Significant Accounting Policies [Line Items]
Estimated useful life in years										20 years
Maximum | Customer relationships [member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Customer relationship estimated life											20 years