Consolidated Statements of Cash Flows - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Cash flows from operating activities:
Net income (loss)	$ 592.7	$ 633.7	$ (334.3)
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Share-based compensation expense	224.6	217.3	240.0
Depreciation, amortization, and accretion	206.7	176.5	186.1
Non-cash restructuring and other (benefits) charges	0.0	(3.5)	139.2
Deferred income taxes	55.9	(14.6)	(16.9)
Impairment of goodwill	0.0	0.0	850.0
Gain on sale of Junos Pulse	0.0	0.0	(19.6)
Loss (gain) on investments and fixed assets, net	3.5	(6.4)	(166.2)
Gain on legal settlement, net	0.0	0.0	(121.1)
Excess tax benefits from share-based compensation	(6.7)	(12.3)	(9.4)
Changes in operating assets and liabilities, net of effects from acquisitions:
Accounts receivable, net	(263.5)	(218.9)	(16.8)
Prepaid expenses and other assets	(43.6)	(43.5)	(10.1)
Accounts payable	66.6	(80.2)	38.3
Accrued compensation	(18.1)	46.6	(46.0)
Income taxes payable	3.1	104.3	51.0
Other accrued liabilities	(16.9)	1.2	(45.9)
Deferred revenue	301.7	92.3	45.1
Net cash provided by operating activities	1,106.0	892.5	763.4
Cash flows from investing activities:
Purchases of property and equipment	(214.7)	(210.3)	(192.9)
Proceeds from sale of Junos Pulse	0.0	0.0	105.7
Purchases of available-for-sale investments	(1,598.0)	(1,486.4)	(2,440.7)
Proceeds from sales of available-for-sale investments	1,182.1	861.6	2,627.7
Proceeds from maturities and redemptions of available-for-sale investments	342.3	319.8	337.6
Purchases of trading investments	(4.9)	(4.4)	(4.1)
Proceeds from the sales of privately-held investments	9.5	10.6	4.9
Purchases of privately-held investments	(20.3)	(5.4)	(21.7)
Payments for business acquisitions, net of cash and cash equivalents acquired	(144.6)	(3.5)	(27.1)
Changes in restricted cash	(1.5)	9.3	44.6
Net cash (used in) provided by investing activities	(450.1)	(508.7)	434.0
Cash flows from financing activities:
Proceeds from issuance of common stock	62.3	121.2	159.8
Purchases and retirement of common stock	(324.6)	(1,152.8)	(2,262.5)
Issuance of long-term debt, net	494.0	594.6	346.5
Payment of long-term debt	(300.0)	0.0	0.0
Payment of financing obligations	(15.5)	(0.4)	(0.4)
Customer financing arrangements	0.0	0.0	9.0
Excess tax benefits from share-based compensation	6.7	12.3	9.4
Payment of dividends	(152.5)	(156.3)	(86.0)
Net cash used in financing activities	(229.6)	(581.4)	(1,824.2)
Effect of foreign currency exchange rates on cash and cash equivalents	(14.0)	(21.1)	(17.6)
Net increase (decrease) in cash and cash equivalents	412.3	(218.7)	(644.4)
Cash and cash equivalents at beginning of period	1,420.9	1,639.6	2,284.0
Cash and cash equivalents at end of period	1,833.2	1,420.9	1,639.6
Supplemental disclosures of cash flow information:
Cash paid for interest, net of amounts capitalized	92.8	80.6	44.9
Cash paid (received) for income taxes, net	173.9	128.3	206.0
Non-cash investing and financing activities:
Construction costs for building with financing obligation	15.3	45.6	0.0
Receipt of a promissory note in connection with the sale of Junos Pulse	$ 0.0	$ 0.0	$ 125.0