Consolidated Statements of Cash Flows - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows from operating activities:
Net income	$ 306.2	$ 592.7	$ 633.7
Adjustments to reconcile net income to net cash provided by operating activities:
Share-based compensation expense	187.5	224.6	217.3
Depreciation, amortization, and accretion	225.6	206.7	176.5
Non-cash restructuring benefits	0.0	0.0	(3.5)
Deferred income taxes	(139.6)	55.9	(14.6)
(Gain) loss on investments and other, net	(14.5)	3.5	(6.4)
Changes in operating assets and liabilities, net of effects from acquisitions:
Accounts receivable, net	203.8	(263.5)	(218.9)
Prepaid expenses and other assets	43.0	(43.6)	(43.5)
Accounts payable	(10.1)	66.6	(80.2)
Accrued compensation	(42.8)	(21.1)	43.7
Income taxes payable	447.3	3.1	104.3
Other accrued liabilities	(1.3)	(19.4)	(1.2)
Deferred revenue	55.0	301.7	92.3
Net cash provided by operating activities	1,260.1	1,107.2	899.5
Cash flows from investing activities:
Purchases of property and equipment	(151.2)	(214.7)	(210.3)
Purchases of available-for-sale investments	(1,882.9)	(1,598.0)	(1,486.4)
Proceeds from sales of available-for-sale investments	944.0	1,182.1	861.6
Proceeds from maturities and redemptions of available-for-sale investments	741.6	342.3	319.8
Purchases of trading investments	(4.6)	(4.9)	(4.4)
Proceeds from sales of trading investments	2.3	3.0	2.9
Proceeds from Pulse note receivable	75.0	0.0	0.0
Purchases of privately-held investments	(10.3)	(20.3)	(5.4)
Proceeds from the sales of privately-held investments	10.1	9.5	10.6
Payments for business acquisitions, net of cash and cash equivalents acquired	(33.0)	(144.6)	(3.5)
Changes in restricted cash	0.0	1.0	11.7
Net cash used in investing activities	(309.0)	(444.6)	(503.4)
Cash flows from financing activities:
Repurchase and retirement of common stock	(725.8)	(324.6)	(1,152.8)
Proceeds from issuance of common stock	64.5	62.3	121.2
Payment of dividends	(150.4)	(152.5)	(156.3)
Customer financing arrangement	16.9	0.0	0.0
Payment of debt	0.0	(300.0)	0.0
Issuance of long-term debt, net	0.0	494.0	594.6
Payment of financing obligations	0.0	(15.5)	(0.4)
Net cash used in financing activities	(794.8)	(236.3)	(593.7)
Effect of foreign currency exchange rates on cash and cash equivalents	17.0	(14.0)	(21.1)
Net increase (decrease) in cash and cash equivalents	173.3	412.3	(218.7)
Cash and cash equivalents at beginning of period	1,833.2	1,420.9	1,639.6
Cash and cash equivalents at end of period	2,006.5	1,833.2	1,420.9
Supplemental disclosures of cash flow information:
Cash paid for interest, net of amounts capitalized	93.9	92.8	80.6
Cash paid (received) for income taxes, net	193.5	173.9	128.3
Non-cash investing and financing activities:
Construction costs for building with financing obligation	$ 0.0	$ 15.3	$ 45.6