BASIS OF PRESENTATION AND SIGNIFICANT ACCOUNTING POLICIES (Details) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								4 Months Ended	8 Months Ended	12 Months Ended
	Jun. 30, 2019 USD ($) $ / shares	Mar. 31, 2019 USD ($) $ / shares	Dec. 31, 2018 USD ($) $ / shares	Sep. 30, 2018 USD ($) $ / shares	Jun. 30, 2018 USD ($) $ / shares	Mar. 31, 2018 USD ($) $ / shares	Dec. 31, 2017 USD ($) $ / shares	Sep. 30, 2017 USD ($) $ / shares	Jun. 30, 2018 USD ($)	Feb. 28, 2018	Jun. 30, 2019 USD ($) segment $ / shares shares	Jun. 30, 2018 USD ($) $ / shares shares	Jun. 30, 2017 USD ($) $ / shares shares
Basis Of Presentation And Significant Accounting Policies [Line Items]
Number of segments | segment											4
Weighted-average shares included to account for the dilutive effect of the Part A and Part B RSUs | shares											1.7	1.2	2.9
Restricted cash, non-current	$ 1.2				$ 4.6				$ 4.6		$ 1.2	$ 4.6	$ 4.5
Depreciation expense											(538.3)	(649.3)	(526.2)
Net income	$ 63.0	$ 34.7	$ 30.2	$ 22.1	$ 42.9	$ 23.5	$ 13.2	$ 23.3			$ 150.0	$ 102.9	$ 87.1
Basic earnings per share | $ / shares	$ 0.27	$ 0.15	$ 0.13	$ 0.09	$ 0.17	$ 0.09	$ 0.05	$ 0.09			$ 0.63	$ 0.42	$ 0.36
Diluted earnings per share | $ / shares	$ 0.26	$ 0.15	$ 0.13	$ 0.09	$ 0.17	$ 0.09	$ 0.05	$ 0.09			$ 0.62	$ 0.41	$ 0.35
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	$ 9,334.6				$ 9,209.9				9,209.9		9,334.6	9,209.9
Liabilities of parent company	7,993.1				7,709.6				7,709.6		7,993.1	7,709.6
Parent Company [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Assets of parent company	0.0										0.0
Liabilities of parent company	0.0										0.0
Cash of parent company	$ 0.7				$ 0.7				$ 0.7		$ 0.7	$ 0.7
Allstream [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Percentage of entity acquired	100.00%										100.00%
Credit Concentration Risk [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Major customer concentration											10.00%	10.00%
Property Plant and Equipment Depreciation Period
Basis Of Presentation And Significant Accounting Policies [Line Items]
Depreciation expense											$ 208.8	$ 69.6
Net income											$ 178.7	$ 55.4
Basic earnings per share | $ / shares											$ 0.75	$ 0.22
Diluted earnings per share | $ / shares											$ 0.74	$ 0.22
Fiber Optic Network [Member] | Property Plant and Equipment Depreciation Period
Basis Of Presentation And Significant Accounting Policies [Line Items]
Estimated useful life in years									33 years
Owned Buildings [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Estimated useful life in years										39 years
Owned Buildings [Member] | Property Plant and Equipment Depreciation 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											14 years 3 months 18 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%
Minimum | Fiber Optic Network [Member]
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 [Member]
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