Stock compensation (Tables)

6 Months Ended

Jun. 30, 2019

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 Six Months Ended				For the Three Months Ended				For the Six Months Ended
	June 30,								June 30,
	2019		2018		2019		2018		2019		2018		2019		2018
Average risk-free interest rates	2.26	%	2.71	%	2.49	%	2.47	%	1.75	%	2.52	%	2.10	%	2.50	%
Average expected life (in years)	5.55		5.37		5.34		5.50		0.50		0.50		0.50		0.50
Volatility	39	%	46	%	45	%	45	%	37	%	30	%	34	%	51	%
Weighted-average fair value (in dollars)	30.92		28.65		31.47		39.47		13.11		12.11		13.69		16.25

Schedule of option activity under the 2010 Stock Plan

Option activity under the 2010 Stock Plan was as follows:

	Shares Subject to
	Outstanding Options
		Weighted Average
	Shares	Exercise Price
Balance at December 31, 2018	12,285,159	$	74.39
Options granted	2,429,688	$	73.23
Options exercised	(1,178,225)	$	26.93
Options cancelled	(567,093)	$	91.76
Balance at June 30, 2019	12,969,529	$	77.72

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
	Outstanding Awards
	Shares	Grant Date Value
Balance at December 31, 2018	2,043,337	$	80.35
RSUs granted	193,950	$	74.74
PSUs granted	100,000	$	74.38
PSUs achieved	44,940	$	65.76
RSUs cancelled	(83,723)	$	85.58
PSUs cancelled	(22,764)	$	66.44
RSUs released	(316,325)	$	87.99
Balance at June 30, 2019	1,959,415	$	77.86

Summary of share activity

The following table summarizes our shares available for grant:

	Shares Available
	for Grant
Balance at December 31, 2018	7,023,328
Additional authorization	7,700,000
Options, RSUs and PSUs granted	(2,891,106)
Options, RSUs and PSUs cancelled	673,580
Balance at June 30, 2019	12,505,802