Segments (Revenue) (Details) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2017 USD ($) Customer	Dec. 31, 2016 USD ($) Customer	Dec. 31, 2015 USD ($) Customer
Net Revenues [Abstract]
Total product									$ 3,446.2	$ 3,528.9	$ 3,563.1
Total service									1,581.0	1,461.2	1,294.7
Total net revenues	$ 1,239.5	$ 1,257.8	$ 1,308.9	$ 1,221.0	$ 1,385.6	$ 1,285.3	$ 1,221.3	$ 1,097.9	$ 5,027.2	$ 4,990.1	$ 4,857.8
Number of customers accounting for more than 10% of revenues | Customer									0	0	0
Routing
Net Revenues [Abstract]
Total product									$ 2,189.5	$ 2,352.9	$ 2,359.2
Switching
Net Revenues [Abstract]
Total product									963.4	858.0	768.3
Security
Net Revenues [Abstract]
Total product									293.3	318.0	435.6
Cloud
Net Revenues [Abstract]
Total net revenues									1,314.9	1,322.3	1,021.2
Telecom/Cable
Net Revenues [Abstract]
Total net revenues									2,315.7	2,324.7	2,417.1
Strategic Enterprise
Net Revenues [Abstract]
Total net revenues									$ 1,396.6	$ 1,343.1	$ 1,419.5