DEFERRED REVENUE - Schedule of Changes in Deferred Revenue (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Contract With Customer, Liability [Roll Forward]
Beginning balance	$ 67.0
Revenue recognized from beginning balance	(3.8)
Increase in deferred revenue (gross)	9.7	$ 2.6
Ending balance	83.8
Prepaid Services, IRUs, And Deferred Non-Recurring Revenue
Contract With Customer, Liability [Roll Forward]
Beginning balance	333.5
Revenue recognized from beginning balance	(28.2)
Increase in deferred revenue (gross)	60.1
Revenue recognized on increase in deferred revenue	(22.2)
Foreign currency translation adjustments	(4.2)
Ending balance	339.0
Less than 1 Year | Prepaid Services, IRUs, And Deferred Non-Recurring Revenue
Contract With Customer, Liability [Roll Forward]
Beginning balance	22.5
Revenue recognized from beginning balance	(17.9)
Increase in deferred revenue (gross)	49.2
Revenue recognized on increase in deferred revenue	(22.0)
Foreign currency translation adjustments	(0.1)
Ending balance	31.7
Greater than 1 Year | Prepaid Services, IRUs, And Deferred Non-Recurring Revenue
Contract With Customer, Liability [Roll Forward]
Beginning balance	311.0
Revenue recognized from beginning balance	(10.3)
Increase in deferred revenue (gross)	10.9
Revenue recognized on increase in deferred revenue	(0.2)
Foreign currency translation adjustments	(4.1)
Ending balance	$ 307.3