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Equity Award Plan Activities (Tables)

12 Months Ended

Dec. 31, 2018

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

The following table summarizes the option activities under our stock plans and related information (in thousands, except years and per share amounts):



	Number of Shares Underlying Outstanding Options		Weighted- Average Exercise Price per Share		Weighted- Average Remaining Contractual Term (In Years)	Aggregate Intrinsic Value
Balance—December 31, 2017	7,024		$	33.05	6.1	$	1,422,637
Options granted	113		241.92
Options exercised	(1,189	)	32.24
Options canceled	(49	)	49.53
Balance—December 31, 2018	5,899		$	37.09	5.2	$	1,027,741
Vested and exercisable—December 31, 2018	3,097		$	25.22	4.7	$	574,392

Schedule of Restricted Stock Units Activity

A summary of the RSU activities under our 2014 Plan and changes during the reporting period and a summary of related information are presented below (in thousands, except years and per share amounts):



	Number of Shares		Weighted- Average Grant Date Fair Value Per Share		Weighted-Average Remaining Contractual Term (In Years)	Aggregate Intrinsic Value
Unvested balance—December 31, 2017	1,537		$	104.29	1.6	$	362,119
RSUs granted	378		257.91
RSUs vested	(538	)	96.49
RSUs forfeited/canceled	(69	)	129.00
Unvested balance—December 31, 2018	1,308		$	150.60	1.5	$	275,638

Schedule of Shares Available for Grant

The following table presents the stock activities and the total number of shares available for grant as of December 31, 2018 (in thousands):



	Number of Shares
Balance—December 31, 2017	13,512
Authorized	2,211
Options granted	(113	)
RSUs granted	(378	)
Options canceled	49
RSUs forfeited	69
Shares traded for taxes	36
Balance—December 31, 2018	15,386

Schedule of Stock-Based Compensation Expense

Total stock-based compensation expense related to options, RSAs, ESPP and RSUs granted were charged to the department to which the associated employee reported as follow (in thousands):



	Year Ended December 31,
	2018		2017		2016
Cost of revenue	$	5,087	$	4,353	$	3,620
Research and development	48,205		42,184		31,892
Sales and marketing	24,995		17,953		15,666
General and administrative	12,915		10,937		7,854
Total stock-based compensation	$	91,202	$	75,427	$	59,032

Schedule of Stock Option Valuation Assumptions

For the years ended December 31, 2018, 2017 and 2016, the fair value of each stock option granted under our plans was estimated on the date of grant using the Black-Scholes option pricing model with the following assumptions:



	Year Ended December 31,
	2018		2017		2016
Expected term (in years)	7.0		6.3		6.7
Risk-free interest rate	2.9	%	2.1	%	1.5	%
Expected volatility	44.6	%	38.9	%	38.9	%
Dividend rate	—	%	—	%	—	%

Schedule of ESPP Valuation Assumptions

The following table summarizes the assumptions relating to our ESPP:



	Year Ended December 31,
	2018		2017		2016
Expected term (in years)	1.1		1.2		1.2
Risk-free interest rate	2.4	%	1.1	%	0.6	%
Expected volatility	41.9	%	31.7	%	31.8	%
Dividend rate	—	%	—	%	—	%

Schedule of Unrecognized Stock-Based Compensation Expense

As of December 31, 2018, unrecognized stock-based compensation expenses by award type and their expected weighted-average recognition periods are summarized in the following table (in thousands, except years).



	December 31, 2018
	Stock Option		RSU		ESPP		Restricted Stock
Unrecognized stock-based compensation expense	$	56,441	$	177,382	$	6,474	$	5,387
Weighted-average amortization period	3.6 years		3.3 years		1.0 year		3.7 years