Stock compensation (Tables)

6 Months Ended

Jun. 30, 2018

Stock compensation

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

	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,
	2018		2017		2018		2017		2018		2017		2018		2017
Average risk-free interest rates	2.71	%	1.56	%	2.47	%	1.79	%	2.52	%	1.38	%	2.50	%	1.31	%
Average expected life (in years)	5.37		5.41		5.50		5.36		0.50		0.50		0.50		0.50
Volatility	46	%	50	%	45	%	49	%	30	%	42	%	51	%	42	%
Weighted-average fair value (in dollars)	28.65		60.44		39.47		52.87		12.11		17.55		16.25		18.80

Schedule of option activity under the 2010 Stock Plan

		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	(1,565,286)	1,565,286	$	89.67
Options exercised	—	(844,619)	$	22.03
Options cancelled	244,146	(244,146)	$	100.67
Balance at June 30, 2018	7,588,561	11,683,074	$	73.89

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

		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	(322,593)	322,593	$	73.90
PSUs granted	(446,500)	446,500	$	65.76
RSUs cancelled	43,588	(43,588)	$	103.70
RSUs released	—	(176,590)	$	81.70
Balance at June 30, 2018	1,043,697	1,727,575	$	87.29

Schedule of summary of share activity

	Shares Issued
	and Outstanding
Balance at December 31, 2017	211,262,906
Exercise of stock options and issuance under ESPP	997,470
Settlement of employee restricted stock units	148,199
Conversion of convertible senior notes	577
Balance at June 30, 2018	212,409,152