Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2018

Equity [Abstract]

Summary of Common Stock Reserved for Future Issuance

Common stock reserved for future issuance as of December 31, 2018 was as follows:

	December 31,
	2018
Outstanding stock options and restricted stock units		3,539,863
Reserved for grants of future stock options and restricted stock units		2,980,762
Reserved for employee stock purchase plan		1,023,970
Total common stock reserved for future issuance		7,544,595

Summary of Shares Available for Grant

A summary of shares available for grant under the Stock Plans is as follows:

	Shares Available
	for Grant
Balance at December 31, 2015		1,856,709
Additional shares reserved		1,125,742
Options and restricted stock granted		(856,043	)
Options and restricted stock cancelled		75,831
Balance at December 31, 2016		2,202,239
Additional shares reserved		1,155,474
Shares forfeited for tax		13,094
Options and restricted stock granted		(1,002,063	)
Options and restricted stock cancelled		97,502
Balance at December 31, 2017		2,466,246
Additional shares reserved		1,189,502
Shares forfeited for tax		28,107
Options and restricted stock granted		(805,653	)
Options and restricted stock cancelled		102,560
Balance at December 31, 2018		2,980,762

Summary of Activity under Stock Plans

A summary of stock option activity under the Stock Plans is as follows:

	Options Outstanding					Weighted Average
	Number of			Weighted Average		Remaining	Aggregate
	Options			Exercise Price		Contractual Term	Intrinsic Value
						(in years)	(in thousands)
Outstanding at December 31, 2015		3,050,288		$	19.74	7.8	$	145,721
Options granted		498,564		$	66.74
Options exercised		(667,494	)	$	10.19		$	46,529
Options cancelled		(60,131	)	$	40.89
Outstanding at December 31, 2016		2,821,227		$	29.85	7.4	$	123,425
Options granted		503,690		$	74.71
Options exercised		(707,410	)	$	10.59		$	50,971
Options cancelled		(57,535	)	$	46.83
Outstanding at December 31, 2017		2,559,972		$	43.62	7.3	$	71,120
Options granted		236,900		$	47.96
Options exercised		(247,768	)	$	16.05		$	15,468
Options cancelled		(19,364	)	$	44.70
Outstanding at December 31, 2018		2,529,740		$	46.72	6.7	$	21,866
Options exercisable as of December 31, 2018		2,478,361		$	46.40	6.7	$	21,866
Options vested, exercisable or expected to    vest as of December 31, 2018		1,700,383		$	38.33	5.8	$	21,866

Summary of Options Outstanding and Vested under Stock Plans by Exercise Price

The options outstanding and vested under the Stock Plans by exercise price, at December 31, 2018, were as follows:

	Options Outstanding					Options Vested
			Weighted Average
			Remaining
	Number		Contractual Term	Weighted Average		Number		Weighted Average
Exercise Price	Outstanding		(in years)	Exercise Price		Exercisable		Exercise Price
$1.44 — $18.00		717,786	4.29	$	8.62		717,786	$	8.62
$32.51 — $53.70		517,192	7.32	$	43.87		315,285	$	44.40
$54.50 — $63.23		543,534	6.96	$	58.63		362,733	$	59.17
$63.39 — $76.81		591,830	8.59	$	73.13		221,697	$	72.83
$86.90 — $97.52		159,398	7.89	$	88.84		82,882	$	89.05
$1.44 — $97.52		2,529,740	6.72	$	46.72		1,700,383	$	38.33

Summary of Restricted Stock Units under the Stock Plan

A summary of RSUs activity under the Stock Plans was as follows:

	Number of			Weighted Average		Aggregate
	Restricted			Grant Date		Intrinsic Value
	Stock Units			Fair Value		(in thousands)
Outstanding at December 31, 2015		5,160		$	56.09	$	348
Restricted stock granted		357,479		$	70.31
Restricted stock released		(1,384	)	$	56.65	$	115
Restricted stock cancelled		(15,700	)	$	65.15
Outstanding at December 31, 2016		345,555		$	70.39	$	25,108
Restricted stock granted		498,373		$	82.60
Restricted stock released		(83,121	)	$	69.90	$	1,175
Restricted stock cancelled		(39,967	)	$	81.57
Outstanding at December 31, 2017		720,840		$	78.26	$	49,767
Restricted stock granted		568,753		$	68.09
Restricted stock released		(196,274	)	$	77.43	$	968
Restricted stock cancelled		(83,196	)	$	81.73
Outstanding at December 31, 2018		1,010,123		$	72.41	$	39,284
Restricted stock expected to vest as of    December 31, 2018		930,205				$	36,176

Schedule of Employee Stock Purchase Plan Activity

A summary of ESPP activity was as follows:

	December 31,
	2018		2017		2016
Additional shares reserved		297,375		288,868		281,435
Shares issued		110,040		73,262		72,568
Shares available for future issuance		1,023,970		836,635		621,029
Employee contributions for shares issued (in thousands)	$	5,521	$	4,697	$	3,499

Schedule of Fair Value of Employee Stock Options Using Black-Scholes Option Valuation Model

The following assumptions were used in estimating the fair value:

	Years Ended December 31,
	2018	2017		2016
Stock Options:
Expected term (in years)	5.5	5.5		5.3 — 6.1
Expected volatility	44% — 51%	44% — 46%		47% — 49%
Risk-free interest rate	2.3% — 2.9%	1.8% — 2.2%		1.3% — 1.9%
Dividend Yield	0%		0%		0%
ESPP:
Expected term (in years)	0.5	0.5		0.5
Expected volatility	44% — 57%	36% — 37%		46% — 53%
Risk-free interest rate	2.1% — 2.5%	1.0% — 1.4%		0.4% — 0.6%
Dividend Yield	0%		0%	0%

Summary of Stock-Based Compensation Expense by Line Items in Consolidated Statements of Operations

A summary of pre-tax stock-based compensation expense by line items in the consolidated statements of operations was as follows (in thousands):

	Years Ended December 31,
	2018		2017		2016
Cost of revenue	$	2,656	$	1,878	$	1,094
Research and development		5,871		4,601		3,182
Sales, general and administrative		28,110		19,664		11,484
Total stock-based compensation expense	$	36,637	$	26,143	$	15,760

Summary of Stock-Based Compensation Expense

A summary of pre-tax stock-based compensation expense by category was as follows (in thousands):

	Years Ended December 31,
	2018		2017		2016
Stock options	$	13,765	$	13,412	$	10,832
Restricted stock units		21,006		11,197		3,548
Employee stock purchase plan		1,866		1,534		1,380
Total stock-based compensation expense	$	36,637	$	26,143	$	15,760

Summary of Stock-Based Compensation Expense Not Yet Recognized

As of December 31, 2018, total stock-based compensation expense not yet recognized, net of estimated forfeitures, were as follows:

	Unrecognized		Weighted-Average
	Compensation		Amortization Period
	(in thousands)		(in years)
Stock options	$	19,545		2.5
Restricted stock units		57,310		2.8
Employee stock purchase plan		1,107		0.4