Segments - Revenue (Details) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020 USD ($)	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2020 USD ($) segment	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments | segment									1
Net Revenues [Abstract]
Net revenues:	$ 1,222.6	$ 1,138.2	$ 1,086.3	$ 998.0	$ 1,208.1	$ 1,133.1	$ 1,102.5	$ 1,001.7	$ 4,445.1	$ 4,445.4	$ 4,647.5
Cloud
Net Revenues [Abstract]
Net revenues:									1,081.2	1,059.8	1,049.9
Service Provider
Net Revenues [Abstract]
Net revenues:									1,761.7	1,827.8	2,066.7
Enterprise
Net Revenues [Abstract]
Net revenues:									1,602.2	1,557.8	1,530.9
Routing
Net Revenues [Abstract]
Net revenues:									1,612.1	1,623.2	1,839.7
Switching
Net Revenues [Abstract]
Net revenues:									918.9	901.0	934.4
Security
Net Revenues [Abstract]
Net revenues:									314.0	343.5	333.0
Product
Net Revenues [Abstract]
Net revenues:									2,845.0	2,867.7	3,107.1
Service
Net Revenues [Abstract]
Net revenues:									$ 1,600.1	$ 1,577.7	$ 1,540.4