BUSINESS ACQUISITIONS - Narrative (Details) € in Millions, $ in Millions		1 Months Ended											12 Months Ended
	Jan. 01, 2017 USD ($)	Dec. 31, 2019 USD ($)	Oct. 31, 2018 USD ($) shares	May 31, 2018 USD ($) shares	Mar. 31, 2018 USD ($) shares	Feb. 28, 2018 USD ($)	Feb. 28, 2018 EUR (€)	Dec. 31, 2017 USD ($) shares	Oct. 31, 2017 USD ($)	Sep. 30, 2017 USD ($) shares	Jun. 30, 2017 USD ($)	Jan. 31, 2017 USD ($) shares	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Business Acquisition [Line Items]
Cash paid at closing, incl. working capital													$ 52.6	$ 2,242.7	$ 706.3
Foreign currency hedge notional amount | €							€ 1,260
Foreign currency hedge spot rate						1.23	1.23
Loss recognized upon settlement of the deal-contingent foreign currency hedge arrangement						$ 105.8
Integration related costs													23.0	$ 40.5	$ 19.1
KPN International
Business Acquisition [Line Items]
Cash paid at closing, incl. working capital		$ 53.6
Cash acquired from acquisition		$ 1.5
Access Point
Business Acquisition [Line Items]
Cash paid at closing, incl. working capital			$ 36.3										0.5
Cash acquired from acquisition			$ 1.0
Number of unregistered shares of common stock (in shares) | shares			115,194
Common stock value			$ 4.6
Interoute Communications Holdings S.A.
Business Acquisition [Line Items]
Cash paid at closing, incl. working capital				$ 2,239.3
Cash acquired from acquisition				$ 66.1
Number of unregistered shares of common stock (in shares) | shares				9,589,094
Common stock value				$ 425.0
Debt				$ 27.7
Accelerated Connections, Inc.
Business Acquisition [Line Items]
Cash paid at closing, incl. working capital					$ 35.0
Cash acquired from acquisition					$ 0.8
Number of unregistered shares of common stock (in shares) | shares					79,930
Common stock value					$ 4.2
Custom Connect International B.V.
Business Acquisition [Line Items]
Cash paid at closing, incl. working capital								$ 28.9
Cash acquired from acquisition								$ 0.6
Number of unregistered shares of common stock (in shares) | shares								49,941
Common stock value								$ 2.2
Transbeam, Inc.
Business Acquisition [Line Items]
Cash paid at closing, incl. working capital									$ 26.4
Cash acquired from acquisition									0.8
Holdback consideration									$ 2.0
Pivotal Global Capacity
Business Acquisition [Line Items]
Cash paid at closing, incl. working capital										$ 104.0
Cash acquired from acquisition										$ 4.0
Number of unregistered shares of common stock (in shares) | shares										1,850,000
Common stock value										$ 53.6
Perseus Telecom
Business Acquisition [Line Items]
Cash paid at closing, incl. working capital											$ 37.5
Cash acquired from acquisition											0.1
Liabilities incurred											$ 1.9
Hibernia
Business Acquisition [Line Items]
Cash paid at closing, incl. working capital												$ 529.6
Cash acquired from acquisition												$ 14.6
Number of unregistered shares of common stock (in shares) | shares												3,329,872
Common stock value	$ 75.0											$ 86.1	$ 86.1