Other Financial Information, Other Income (Expense), Net (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Oct. 01, 2014
Other Financial Information [Abstract]
Interest income									$ 21.8	$ 10.0	$ 8.7
Interest expense									(83.3)	(66.9)	(58.4)
Gain on investments									6.8	167.9	11.3
Gain on sale of Junos Pulse									0.0	19.6	0.0
Other									(5.1)	6.7	(2.0)
Other (expense) income, net	$ (18.5)	$ (8.4)	$ (17.1)	$ (15.8)	$ 7.4	$ (6.8)	$ 178.6	$ 154.2	(59.8)	333.4	(40.4)
Interest expense, long-term debt									79.8	57.5	45.2
Capitalized interest									2.2	2.7	1.9
Cost-method investments, realized gains (loss)	7.3								6.8		7.1
Publicly-traded equity and privately-held investments, net realized gain								$ 163.0		163.0
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net gain on legal settlement						$ 0.8	$ 195.3		0.0	196.1	$ 0.0
Junos Pulse [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Consideration												$ 230.7
Cash consideration												105.7
Working capital adjustment												19.3
Promissory note in connection with the sale of Junos Pulse	$ 132.9				$ 125.0				$ 132.9	125.0		$ 125.0
Palo Alto Networks [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net gain on legal settlement										$ 196.1