6. Equity Award Plan Activities

2014 Equity Incentive Plan

Effective January 1, 2018, our board of directors authorized an increase of 2,211,176 shares to the shares available for issuance under the 2014 Equity Incentive Plan (the “2014 Plan”). Pursuant to the 2014 Plan, the 2018 share increase is determined based on the lesser of 3% of total shares of common stock outstanding as of December 31, 2017, 12,500,000 shares, or such amount as determined by our board of directors. As of March 31, 2018, there remained approximately 23.8 million shares available for issuance under the 2014 Plan.

2014 Employee Stock Purchase Plan

Effective January 1, 2018, our board of directors authorized an increase of 737,058 shares to shares available for issuance under our 2014 Employee Stock Purchase Plan (the “ESPP”). Pursuant to the ESPP, the 2018 share increase is determined based the lesser of 1% of the total shares of common stock outstanding on December 31, 2017, 2,500,000 shares, or such amount as determined by our board of directors. As of March 31, 2018, there remained approximately 2,615,207 shares available for issuance under the ESPP.

During the three months ended March 31, 2018, we issued 108,890 shares at a weighted-average purchase price of $67.09 under the ESPP.

Stock Option Activities

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



	Options Outstanding
	Number of Shares Underlying Outstanding Options		Weighted- Average Exercise Price per Share		Weighted- Average Remaining Contractual Term (Years) of Stock Options	Aggregate Intrinsic Value of Stock Options Outstanding
Balance—December 31, 2017	7,024		$	33.05	6.1	$	1,422,637
Options granted	—		—
Options exercised	(394	)	25.39
Options canceled	(18	)	41.58
Balance—March 31, 2018	6,612		$	33.48	5.9	$	1,466,783
Vested and exercisable—March 31, 2018	2,911		$	22.71	5.3	$	676,982

Restricted Stock Unit (RSU) Activities

A summary of the RSU activity under our stock plans and 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	153		279.87
RSUs vested	(136	)	87.07
RSUs forfeited/canceled	(25	)	121.92
Unvested balance—March 31, 2018	1,529		$	123.10	1.6	$	390,315

Shares Available for Grant

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



	Number of Shares
Balance—December 31, 2017	13,512
Authorized	2,211
RSUs granted	(153	)
Options canceled	18
RSUs forfeited	25
Shares traded for taxes	7
Balance—March 31, 2018	15,620

Stock-Based Compensation Expense

Total stock-based compensation expense related to options, restricted stock awards, restricted stock units and employee stock purchase rights granted were allocated as follows (in thousands):



	Three Months Ended March 31,
	2018		2017
Cost of revenue	$	1,202	$	1,024
Research and development	10,945		9,587
Sales and marketing	5,960		3,456
General and administrative	2,744		2,372
Total stock-based compensation	$	20,851	$	16,439

As of March 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).



	March 31, 2018
	Stock Option		RSU		ESPP
Unrecognized stock-based compensation expense	$	61,271	$	174,249	$	4,996
Weighted-average amortization period	3.5 years		3.5 years		1.1 years