Commitment and Contingencies - MTA Agreement (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Change In MTA Deployment Costs [Roll Forward]
Prepaid MTA equipment deployment costs, beginning of period				$ 79.5				$ 4.7	$ 79.5	$ 4.7
Intangible assets, gross, beginning of period				1,625.0					1,625.0
MTA deployment costs, beginning of period				94.3				5.6	94.3	5.6
Recoupment									(32.2)	(1.7)
Amortization of intangible assets	$ 26.2	$ 28.7	$ 27.6	24.7	$ 25.8	$ 25.8	$ 25.0	22.5	107.2	99.1	$ 100.1
MTA Equipment Development Cost Additions									150.8	91.2
Prepaid MTA equipment deployment costs, end of period	171.5				79.5				171.5	79.5	4.7
Intangible assets, gross, end of period	1,697.8				1,625.0				1,697.8	1,625.0
MTA deployment costs, end of period	209.8				94.3				209.8	94.3	5.6
MTA equipment deployment costs
Change In MTA Deployment Costs [Roll Forward]
Deployment costs incurred									124.2	76.5
Recoupment									(32.2)	(1.7)
Amortization of intangible assets									(3.1)	(0.8)
Franchise agreements
Change In MTA Deployment Costs [Roll Forward]
Intangible assets, gross, beginning of period				470.7					470.7
Intangible assets, gross, end of period	497.4				470.7				497.4	470.7
Franchise agreements | MTA equipment deployment costs
Change In MTA Deployment Costs [Roll Forward]
Intangible assets, gross, beginning of period				$ 14.8				$ 0.9	14.8	0.9
Intangible asset additions									26.6	14.7
Amortization of intangible assets									(3.1)	(0.8)
Intangible assets, gross, end of period	$ 38.3				$ 14.8				$ 38.3	$ 14.8	$ 0.9