Stock compensation (Tables)

3 Months Ended

Mar. 31, 2019

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,
	2019		2018		2019		2018
Average risk-free interest rates	2.51	%	2.42	%	2.27	%	2.26	%
Average expected life (in years)	5.33		5.53		0.25		0.25
Volatility	45	%	45	%	34	%	36	%
Weighted-average fair value (in dollars)	31.50		41.59		13.31		18.17

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, 2018	6,477,577	12,285,159	$	74.39
Options granted	(2,271,592)	2,271,592	$	72.81
Options exercised	—	(849,870)	$	25.38
Options cancelled	315,321	(315,321)	$	89.29
Balance at March 31, 2019	4,521,306	13,391,560	$	76.88

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, 2018	545,751	2,043,337	$	80.35
RSUs granted	(165,251)	165,251	$	73.69
PSUs achieved	(44,940)	44,940	$	65.76
RSUs cancelled	55,591	(55,591)	$	89.46
PSUs cancelled	9,889	(9,889)	$	67.32
RSUs released	—	(213,604)	$	89.38
Balance at March 31, 2019	401,040	1,974,444	$	78.29