ACQUISITIONS AND DISPOSITIONS (Schedule of Acquisition) (Details) $ in Millions, $ in Millions	Apr. 17, 2018 USD ($)	Apr. 04, 2018 USD ($)	Feb. 28, 2018 USD ($)	Jan. 18, 2018 CAD ($)	Jan. 18, 2018 USD ($)	May 01, 2017 USD ($)	Mar. 01, 2017 USD ($)	Oct. 03, 2016 USD ($)	Apr. 01, 2016 USD ($)	Jan. 15, 2016 CAD ($)	Jan. 15, 2016 USD ($)	Dec. 31, 2015 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)
Business Acquisition [Line Items]
Goodwill													$ 1,719.1	$ 1,840.2	$ 1,214.5
Neutral path communications and near north partners
Business Acquisition [Line Items]
Cash	$ 0.7
Other current assets	0.1
Property and equipment	10.6
Intangibles	3.6
Goodwill	23.8
Deferred tax assets, net	1.5
Total assets acquired	40.3
Current liabilities	0.6
Deferred revenue	5.7
Total liabilities assumed	6.3
Net assets acquired	34.0
Less cash acquired	(0.7)
Total consideration paid/payable	$ 33.3
McLean Data Center
Business Acquisition [Line Items]
Cash		$ 9.2
Property and equipment		0.6
Total assets acquired		9.8
Current liabilities		1.6
Other liabilities		8.2
Total liabilities assumed		9.8
Less cash acquired		(9.2)
Total consideration paid/payable		$ (9.2)
Spread Networks
Business Acquisition [Line Items]
Cash			$ 1.5
Other current assets			5.9
Property and equipment			144.7
Intangibles			9.3
Goodwill			13.0
Deferred tax assets, net			7.1
Other assets			1.4
Total assets acquired			182.9
Current liabilities			3.8
Deferred revenue			27.2
Other liabilities			19.9
Total liabilities assumed			50.9
Net assets acquired			132.0
Less cash acquired			(1.5)
Total consideration paid/payable			$ 130.5
Optic Zoo Networks
Business Acquisition [Line Items]
Cash					$ 1.4
Other current assets					0.4
Property and equipment					13.6
Intangibles					2.8
Goodwill					10.6
Other assets					0.2
Total assets acquired					29.0
Current liabilities					0.6
Deferred revenue					1.2
Deferred tax liability, net					1.0
Total liabilities assumed					2.8
Net assets acquired					26.2
Less cash acquired					(1.4)
Total consideration paid/payable				$ 30.9	$ 24.8
KIO Networks US Data Centers
Business Acquisition [Line Items]
Cash						$ 0.1
Property and equipment						2.4
Intangibles						6.4
Goodwill						2.7
Deferred tax assets, net						2.2
Other assets						0.5
Total assets acquired						14.3
Current liabilities						1.8
Deferred revenue						0.5
Total liabilities assumed						2.3
Net assets acquired						12.0
Less cash acquired						(0.1)
Total consideration paid/payable						$ 11.9
Electric Lightwave
Business Acquisition [Line Items]
Cash							$ 12.6
Other current assets							55.0
Property and equipment							681.3
Intangibles							416.1
Goodwill							467.1
Deferred tax assets, net														$ 46.7
Other assets							1.7
Total assets acquired							1,633.8
Current liabilities							61.7
Deferred revenue							80.0
Deferred tax liability, net							51.7
Other liabilities							1.2
Total liabilities assumed							194.6
Net assets acquired							1,439.2
Less cash acquired							(12.6)
Total consideration paid/payable							$ 1,426.6
Santa Clara Data Center
Business Acquisition [Line Items]
Property and equipment								$ 31.9
Intangibles								6.0
Total assets acquired								37.9
Capital lease obligations								26.6
Total liabilities assumed								26.6
Net assets acquired								11.3
Total consideration paid/payable								$ 11.3
Clearview
Business Acquisition [Line Items]
Other current assets									$ 0.6
Property and equipment									17.1
Intangibles									9.8
Goodwill									2.1
Deferred tax assets, net									0.2
Other assets									0.3
Total assets acquired									30.1
Current liabilities									1.1
Deferred revenue									0.4
Other liabilities									10.3
Total liabilities assumed									11.8
Net assets acquired									18.3
Total consideration paid/payable									$ 18.3
Allstream
Business Acquisition [Line Items]
Cash											$ 2.9
Other current assets											95.6
Property and equipment											266.3
Intangibles											64.5
Deferred tax assets, net											3.8
Other assets											4.5
Total assets acquired											437.6
Current liabilities											63.2
Deferred revenue											46.9
Other liabilities											27.0
Total liabilities assumed											137.1
Net assets acquired											300.5
Less cash acquired											(2.9)
Total consideration paid/payable										$ 422.9	$ 297.6
Viatel
Business Acquisition [Line Items]
Cash												$ 3.5
Other current assets												7.3
Property and equipment												174.0
Goodwill												9.5
Other assets												2.0
Total assets acquired												196.3
Current liabilities												18.8
Deferred revenue												58.5
Deferred tax liability, net												8.6
Other liabilities												5.7
Total liabilities assumed												91.6
Net assets acquired												104.7
Less cash acquired												(3.5)
Total consideration paid/payable												101.2
Dallas Data Center
Business Acquisition [Line Items]
Property and equipment												12.2
Intangibles												4.4
Total assets acquired												16.6
Net assets acquired												16.6
Total consideration paid/payable												$ 16.6