Consolidated Statements of Cash Flows - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Statement of Cash Flows [Abstract]
Net income (loss)	$ 633.7	$ (334.3)	$ 439.8
Adjustments to reconcile consolidated net income to net cash provided by operating activities:
Share-based compensation expense	217.3	240.0	244.6
Depreciation, amortization, and accretion	176.5	186.1	189.9
Restructuring and other charges	(4.1)	208.5	47.5
Deferred income taxes	(14.6)	(16.9)	72.2
Impairment of goodwill	0.0	850.0	0.0
Gain on sale of Junos Pulse	0.0	(19.6)	0.0
(Gain) loss on investments, net	(6.8)	(167.9)	(11.3)
Gain on legal settlement, net	0.0	(121.1)	0.0
Excess tax benefits from share-based compensation	(12.3)	(9.4)	(1.9)
Loss on disposal of fixed assets	0.4	1.7	1.4
Changes in operating assets and liabilities, net of effects from acquisitions:
Accounts receivable, net	(218.9)	(16.8)	(139.9)
Prepaid expenses and other assets	(43.5)	(10.1)	(126.0)
Accounts payable	(80.2)	38.3	(8.9)
Accrued compensation	46.6	(46.0)	(5.4)
Income tax payable	104.3	51.0	(38.5)
Other accrued liabilities	1.8	(115.2)	36.5
Deferred revenue	92.3	45.1	145.9
Net cash provided by operating activities	892.5	763.4	845.9
Cash flows from investing activities:
Purchases of property and equipment	(210.3)	(192.9)	(230.0)
Proceeds from sale of Junos Pulse	0.0	105.7	0.0
Purchases of available-for-sale investments	(1,486.4)	(2,440.7)	(1,776.0)
Proceeds from sales of available-for-sale investments	861.6	2,627.7	1,167.2
Proceeds from maturities of available-for-sale investments	319.8	337.6	334.6
Purchases of trading investments	(4.4)	(4.1)	(3.7)
Proceeds from the sales of privately-held investments	10.6	4.9	9.4
Purchases of privately-held investments	(5.4)	(21.7)	(41.3)
Payment for business acquisitions, net of cash and cash equivalents acquired	(3.5)	(27.1)	(10.0)
Purchase of licensed software	0.0	0.0	(10.0)
Changes in restricted cash	9.3	44.6	(1.2)
Net cash used in investing activities	(508.7)	434.0	(561.0)
Cash flows from financing activities:
Proceeds from issuance of common stock	121.2	159.8	141.7
Purchases and retirement of common stock	(1,153.6)	(2,262.5)	(577.8)
Issuance of long-term debt, net	594.6	346.5	0.0
Payment for capital lease obligation	0.4	(0.4)	(1.4)
Customer financing arrangements	0.0	9.0	33.9
Excess tax benefits from share-based compensation	12.3	9.4	1.9
Payment of cash dividends	(156.3)	(86.0)	0.0
Net cash (used in) provided by financing activities	(581.4)	(1,824.2)	(401.7)
Effect of foreign currency exchange rates on cash and cash equivalents	(21.1)	(17.6)	(7.0)
Net (decrease) increase in cash and cash equivalents	(218.7)	(644.4)	(123.8)
Cash and cash equivalents at beginning of period	1,639.6	2,284.0	2,407.8
Cash and cash equivalents at end of period	1,420.9	1,639.6	2,284.0
Supplemental disclosures of cash flow information:
Cash paid for interest, net of amounts capitalized	80.6	44.9	57.4
Cash paid (received) for income taxes, net	128.3	206.0	105.1
Non-cash investing activities:
Receipt of a promissory note in connection with the sale of Junos Pulse	$ 0.0	$ 125.0	$ 0.0