CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Cash flows from operating activities:
Net income	$ 178.6	$ 235.5
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	7.9	7.7
Share-based compensation benefit	(19.1)	(144.6)
Other	(8.5)	(1.1)
Excess tax benefits from share-based compensation		(2.1)
Changes in operating assets and liabilities:
Accounts receivable	26.5	(2.0)
Inventories	(7.3)	(5.5)
Accounts payable and accrued expenses	(1.1)	0.5
Other assets and liabilities	53.8	37.4
Net cash provided by operating activities	230.8	125.8
Cash flows from investing activities:
Purchases of property, plant and equipment	(15.6)	(3.8)
Purchases of held-to-maturity investments and other	(25.1)
Maturities of held-to-maturity investments	20.8	21.0
Purchases of available-for-sale investments	(145.7)
Intangible assets acquired		(5.2)
Net cash (used in) provided by investing activities	(165.6)	12.0
Cash flows from financing activities:
Principal payments of debt		(0.1)
Payments of debt issuance costs	(0.7)	(6.8)
Payments to repurchase common stock		(123.2)
Proceeds from the exercise of stock options	33.0	2.6
Issuance of stock under employee stock purchase plan	2.1	2.2
Excess tax benefits from share-based compensation		2.1
Net cash provided by (used in) financing activities	34.4	(123.2)
Effect of exchange rate changes on cash and cash equivalents		0.4
Net increase in cash and cash equivalents	99.6	15.0
Cash and cash equivalents, beginning of period	1,023.0	831.8
Cash and cash equivalents, end of period	1,122.6	846.8
Supplemental cash flow information:
Cash paid for interest	1.0	0.1
Cash paid for income taxes		88.6
Non-cash investing and financing activities:
Non-cash additions to property, plant and equipment	$ 6.5	3.3
Issuance of common stock upon conversion of convertible notes		$ 0.1