Stock compensation (Tables)

9 Months Ended

Sep. 30, 2018

Stock compensation

Schedule of valuation assumptions used for valuation of fair value of stock compensation granted

We utilized the Black-Scholes valuation model for estimating the fair value of the stock compensation granted, with the following weighted-average assumptions:

	Employee Stock Options								Employee Stock Purchase Plan
	For the Three Months Ended				For the Nine Months Ended				For the Three Months Ended				For the Nine Months Ended
	September 30,								September 30,
	2018		2017		2018		2017		2018		2017		2018		2017
Average risk-free interest rates	2.72	%	1.81	%	2.60	%	1.79	%	2.81	%	1.47	%	2.63	%	1.37	%
Average expected life (in years)	5.03		4.99		5.27		5.24		0.50		0.50		0.50		0.50
Volatility	45	%	48	%	45	%	49	%	27	%	39	%	45	%	41	%
Weighted-average fair value (in dollars)	29.32		55.80		34.41		53.81		13.56		20.46		15.91		19.89

Schedule of option activity under the 2010 Stock Plan

Option activity under the 2010 Stock Plan was as follows:

		Shares Subject to
		Outstanding Options
	Shares Available		Weighted Average
	for Grant	Shares	Exercise Price
Balance at December 31, 2017	3,909,701	11,206,553	$	68.36
Additional authorization	5,000,000	—		—
Options granted	(3,122,149)	3,122,149	$	79.08
Options exercised	—	(1,080,228)	$	22.89
Options cancelled	495,003	(495,003)	$	96.25
Balance at September 30, 2018	6,282,555	12,753,471	$	73.76

Schedule of RSU award and PSU activity under the 2010 Stock Plan

RSU award and PSU activity under the 2010 Stock Plan was as follows:

		Shares Subject to
	Shares Available	Outstanding Awards
	for Grant	Shares	Grant Date Value
Balance at December 31, 2017	769,202	1,178,660	$	98.88
Additional authorization	1,000,000	—		—
RSUs granted	(867,197)	867,197	$	70.48
PSUs granted	(523,743)	523,743	$	66.18
RSUs cancelled	87,983	(87,983)	$	97.19
PSUs cancelled	6,750	(6,750)	$	65.76
RSUs released	—	(337,357)	$	92.88
Balance at September 30, 2018	472,995	2,137,510	$	80.47

Summary of share activity

The following table summarizes our share activity:

	Shares Issued
	and Outstanding
Balance at December 31, 2017	211,262,906
Exercise of stock options and issuance under ESPP	1,234,321
Settlement of employee restricted stock units	256,182
Conversion of convertible senior notes	615
Balance at September 30, 2018	212,754,024