Acquisitions (Narrative) (Details) € in Millions, CAD in Millions, $ in Millions													1 Months Ended		12 Months Ended
	May 01, 2017 USD ($) ft² Center MW	Mar. 01, 2017 USD ($) building item mi Center	Oct. 03, 2016 USD ($) ft² payment MW	Apr. 01, 2016 USD ($) ft² item	Jan. 15, 2016 CAD building item mi	Jan. 15, 2016 USD ($) building item mi	Dec. 31, 2015 EUR (€) ft²	Dec. 31, 2015 USD ($) ft²	Feb. 23, 2015 USD ($) ft² item MW	Jan. 01, 2015 USD ($) item mi	Jul. 01, 2014 EUR (€)	Jul. 01, 2014 USD ($)	Apr. 30, 2016 EUR (€)	Apr. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) item	Jun. 30, 2016 USD ($)	Jun. 30, 2015 USD ($)	Jul. 31, 2014
Business Acquisition [Line Items]
Number of business combinations completed | item															41
Net consideration paid															$ 1,434.8	$ 437.5	$ 855.7
Acquisition related costs															20.5	21.5	$ 6.2
Revenue															2,566.7	$ 2,574.4
Long Haul Fiber [Member]
Business Acquisition [Line Items]
Additional Route Miles Acquired | mi		8,100
Metro Fiber [Member]
Business Acquisition [Line Items]
Additional Route Miles Acquired | mi		4,000
KIO Networks US Data Centers
Business Acquisition [Line Items]
Acquisition date	May 01, 2017
Business acquisition purchase price	$ 11.9
Purchase price, held in escrow	$ 1.2
Number of data centers | Center	2
Acquired facility size (in square feet) | ft²	100,000
Acquired megawatts of critical power | MW	2
Electric Lightwave
Business Acquisition [Line Items]
Acquisition date		Mar. 01, 2017
Business acquisition purchase price		$ 1,426.6
Purchase price, held in escrow		$ 7.0
Number of data centers | Center		100
Number of metropolitan markets | item		35
Remaining cash consideration to be paid		$ 2.2
Number of on-net buildings connected | building		3,100
Revenue															169.6
Santa Clara Data Center
Business Acquisition [Line Items]
Acquisition date			Oct. 03, 2016
Business acquisition purchase price			$ 11.3
Acquired facility size (in square feet) | ft²			26,900
Acquired megawatts of critical power | MW			3
Remaining cash consideration to be paid			$ 8.9
Number of quarterly payments | payment			10
Amount of quarterly payments			$ 1.3
Net consideration paid			$ 3.7
Clearview
Business Acquisition [Line Items]
Acquisition date				Apr. 01, 2016
Business acquisition purchase price				$ 18.3
Number of data centers | item				2
Acquired facility size (in square feet) | ft²				30,000
Net consideration paid				$ 18.3
Business acquisition, percentage of voting interests acquired				100.00%
Allstream
Business Acquisition [Line Items]
Acquisition date					Jan. 15, 2016	Jan. 15, 2016
Business acquisition purchase price						$ 297.6
Number of metropolitan markets | item					5	5
Number of on-net buildings connected | building					3,300	3,300
Net consideration paid					CAD 422.9	$ 297.6
Business acquisition, percentage of voting interests acquired						100.00%
Working capital and other liabilities assumed in the acquisition						$ 29.6
Other post retirement benefits plan liability assumed						$ 8.3									$ 9.9
Additional Route Miles Acquired | mi					12,500	12,500
Allstream | Minimum
Business Acquisition [Line Items]
Additional Route Miles Acquired | mi					18,000	18,000
Allstream | Metro Fiber [Member]
Business Acquisition [Line Items]
Additional Route Miles Acquired | mi					5,500	5,500
Viatel
Business Acquisition [Line Items]
Acquisition date							Dec. 31, 2015	Dec. 31, 2015
Business acquisition purchase price								$ 101.2
Net consideration paid							€ 92.9	$ 101.2
Business acquisition, percentage of voting interests acquired							100.00%	100.00%
Refund of escrow reflected as a cash inflow from investing activities								$ 1.5
Dallas Data Center
Business Acquisition [Line Items]
Acquisition date							Dec. 31, 2015	Dec. 31, 2015
Business acquisition purchase price								$ 16.6
Acquired area in square feet | ft²							36,000	36,000
Neo Telecoms
Business Acquisition [Line Items]
Acquisition date											Jul. 01, 2014	Jul. 01, 2014
Business acquisition purchase price												$ 73.9
Purchase price, held in escrow											€ 8.7	11.9	€ 2.0	$ 2.3
Business Acquisition Percentage Of Non Controlling Interest																		4.00%
Net consideration paid											€ 54.1	$ 73.9
Business acquisition, percentage of voting interests acquired												96.00%	4.00%	4.00%
Neo Telecoms | Other long-term liabilities
Business Acquisition [Line Items]
Business acquisition, fair value of non-controlling interest												$ 2.9
Atlanta NAP
Business Acquisition [Line Items]
Acquisition date											Jul. 01, 2014	Jul. 01, 2014
Business acquisition purchase price												$ 51.9
Net consideration paid												$ 51.9
Business acquisition, percentage of voting interests acquired												100.00%
Ideatek Systems, Inc
Business Acquisition [Line Items]
Acquisition date										Jan. 01, 2015
Business acquisition purchase price										$ 52.7
Net consideration paid										$ 52.7
Additional Route Miles Acquired | mi										1,800
Number Of Additional Cellular Towers Connected | item										600
Number Of Additional Building connected | item										100
Latisys Holdings, LLC
Business Acquisition [Line Items]
Acquisition date									Feb. 23, 2015
Business acquisition purchase price									$ 677.8
Number of data centers | item									8
Net consideration paid									$ 677.8
Acquired area in square feet | ft²									185,000
Critical Power Acquired | MW									33