Segments - Revenue (Details) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2019 USD ($) segment	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments | segment									1
Net Revenues [Abstract]
Net revenues	$ 1,208.1	$ 1,133.1	$ 1,102.5	$ 1,001.7	$ 1,181.0	$ 1,179.8	$ 1,204.1	$ 1,082.6	$ 4,445.4	$ 4,647.5	$ 5,027.2
Routing
Net Revenues [Abstract]
Net revenues									1,623.2	1,839.7	2,189.5
Switching
Net Revenues [Abstract]
Net revenues									901.0	934.4	963.4
Security
Net Revenues [Abstract]
Net revenues									343.5	333.0	293.3
Product
Net Revenues [Abstract]
Net revenues									2,867.7	3,107.1	3,446.2
Service
Net Revenues [Abstract]
Net revenues									1,577.7	1,540.4	1,581.0
Cloud
Net Revenues [Abstract]
Net revenues									1,059.8	1,049.9	1,310.7
Service Provider
Net Revenues [Abstract]
Net revenues									1,827.8	2,066.7	2,319.4
Enterprise
Net Revenues [Abstract]
Net revenues									$ 1,557.8	$ 1,530.9	$ 1,397.1