Stock Plans and Stock-Based Compensation (Tables)

9 Months Ended

Sep. 30, 2018

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Schedule of Assumptions Used in Determining Fair Value of Stock Option Grants

The fair value of stock option grants and ESPP are determined using the Black-Scholes option pricing model with the following weighted average assumptions. In addition, the fair value per share on grant date is presented below: 

	Three Months Ended September 30,				Nine Months Ended September 30,
	2018		2017		2018		2017
Employee Stock Options:
Market price per share on grant date	$58.75		$23.17 - $23.60		$33.06 - $58.75		$18.05 - $24.87
Expected term (in years)		6.00		6.00		6.00	6.00 - 6.11
Expected volatility	45%		60%		45% - 50%		60%
Risk-free interest rate	2.72%		1.82% - 1.93%		2.72% - 2.98%		1.82% - 2.47%
Dividend rate	0%		0%		0%		0%
Employee Stock Purchase Plan:
Expected term (in years)		0.50		0.50		0.50		0.50
Expected volatility	45% - 50%		60%		45% - 60%		60%
Risk-free interest rate	1.89% - 2.33%		1.18%		1.18% - 2.33%		0.45% - 1.18%
Dividend rate	0%		0%		0%		0%

⁽¹⁾ The expected term represents the period that the stock-based compensation awards are expected to be outstanding. Since the Company did not have sufficient historical information to develop reasonable expectations about future exercise behavior, the Company used the simplified method to compute expected term, which reflects the average of the time-to-vesting and the contractual life;

⁽²⁾ The expected volatility of the Company’s common stock on the date of grant is based on the weighted average of the Company’s historical volatility as a public company and the volatilities of publicly traded peer companies that are reasonably comparable to the Company’s own operations;

⁽³⁾ The risk-free interest rate is based on the U.S. Treasury yield curve in effect at the time of grant for zero coupon U.S. Treasury notes with maturities approximately equal to the expected term of the options; and

⁽⁴⁾ The expected dividend yield is assumed to be zero as the Company has never paid dividends and has no current plans to pay any dividends on the Company’s common stock.

Schedule of Stock Option Activity

A summary of activities under the 2008 Plan and the 2016 Plan is shown as follows for the year ended December 31, 2017 and the nine months ended September 30, 2018:

	Stock options outstanding			Weighted average exercise price
Outstanding at December 31, 2016		1,884,425		$	10.02
Granted		1,096,881			22.84
Exercised		(497,175	)		5.77
Forfeited		(43,841	)		15.60
Outstanding at December 31, 2017		2,440,290			16.55
Granted		557,896			35.01
Exercised		(703,826	)		12.53
Forfeited		(457,689	)		22.38
Outstanding at September 30, 2018		1,836,671		$	22.24

Schedule of Stock Options Outstanding and Options Exercisable and Vested

Stock options outstanding, and options exercisable and vested are as follows:

Outstanding as of September 30, 2018		Remaining contractual life (years)		Weighted average exercise price		Exercisable as of September 30, 2018		Remaining contractual life (years)		Weighted average exercise price
	1,836,671		8.13	$	22.24		515,735		6.89	$	13.51

Outstanding as of December 31, 2017		Remaining contractual life (years)		Weighted average exercise price		Exercisable as of December 31, 2017		Remaining contractual life (years)		Weighted average exercise price
	2,440,290		8.28	$	16.55		748,148		7.00	$	10.39

Schedule of Vested and Nonvested Stock Option Activity

Vested and nonvested stock option activity was as follows:

	Vested				Nonvested
	Options outstanding		Weighted average exercise price		Options outstanding		Weighted average exercise price
Outstanding at September 30, 2018		515,735	$	13.51		1,320,936	$	25.65
Outstanding at December 31, 2017		748,148	$	10.39		1,692,142	$	19.27

Summary of Activity in Connection with RSUs, Market-based RSUs and RSUs

A summary of activity in connection with the Company’s RSUs, market-based RSUs and PSUs for the nine-month period ended September 30, 2018 is as follows:

	Number of Shares
Outstanding as of December 31, 2017		784,000
Granted		424,741
Vested		(681,980	)
Forfeited		(4,313	)
Outstanding as of September 30, 2018		522,448

Summary of Stock-Based Compensation Expense

The Company recorded the total stock-based compensation expense as follows (in thousands):

	Three Months Ended September 30,				Nine Months Ended September 30,
	2018		2017		2018		2017
Cost of revenue	$	312	$	141	$	1,877	$	266
Sales and marketing		1,180		691		7,147		1,250
Research and development		1,091		416		5,606		738
General and administrative		958		1,555		5,627		2,618
Total	$	3,541	$	2,803	$	20,257	$	4,872