Stock compensation (Tables)

3 Months Ended

Mar. 31, 2020

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 March 31,
	2020		2019		2020		2019
Average risk-free interest rates	1.56	%	2.51	%	0.23	%	2.27	%
Average expected life (in years)	4.65		5.33		0.25		0.25
Volatility	40	%	45	%	52	%	34	%
Weighted-average fair value (in dollars)	28.43		31.50		16.00		13.31

Schedule of option activity under the 2010 Stock Plan

Option activity under our 2010 Amended and Restated Stock Incentive Plan (the “2010 Stock Plan”) was as follows:

	Shares Subject to
	Outstanding Options
		Weighted Average
	Shares	Exercise Price
Balance at December 31, 2019	12,632,657	$	81.42
Options granted	914,834	$	80.12
Options exercised	(688,202)	$	26.83
Options cancelled	(180,342)	$	91.57
Balance at March 31, 2020	12,678,947	$	84.14

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

Restricted stock unit (“RSU”) and performance share (“PSU”) award activity under the 2010 Stock Plan was as follows:

	Shares Subject to
	Outstanding Awards
	Shares	Grant Date Value
Balance at December 31, 2019	2,602,376	$	79.69
RSUs granted	379,496	$	80.32
RSUs released	(64,426)	$	87.36
RSUs cancelled	(39,795)	$	80.31
PSUs cancelled	(12,250)	$	65.76
Balance at March 31, 2020	2,865,401	$	79.65

Summary of shares available for grant

The following table summarizes our shares available for grant under the 2010 Stock Plan:

	Shares Available
	for Grant
Balance at December 31, 2019	9,882,122
Options, RSUs and PSUs granted	(1,673,826)
Options, RSUs and PSUs cancelled	247,429
Balance at March 31, 2020	8,455,725