7. 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 September 30, 2018, there remained approximately 22.9 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 September 30, 2018, there remained 2,533,438 shares available for issuance under the ESPP.

During the three and nine months ended September 30, 2018, we issued 81,769 shares and 190,659 shares at a weighted-average purchase price of $98.00 and $80.35 per share, respectively, 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	82		244.20
Options exercised	(1,038	)	33.07
Options canceled	(41	)	53.44
Balance—September 30, 2018	6,027		$	35.78	5.4	$	1,386,807
Vested and exercisable—September 30, 2018	2,734		$	22.39	4.8	$	665,576

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	231		274.46
RSUs vested	(405	)	93.92
RSUs forfeited/canceled	(56	)	127.57
Unvested balance—September 30, 2018	1,307		$	136.63	1.5	$	347,386

Shares Available for Grant

The following table presents the stock activity and the total number of shares available for grant under the 2014 Plan as of September 30, 2018 (in thousands):



	Number of Shares
Balance—December 31, 2017	13,512
Authorized	2,211
Options granted	(82	)
RSUs granted	(231	)
Options canceled	41
RSUs forfeited	56
Shares traded for taxes	27
Balance—September 30, 2018	15,534

Stock-Based Compensation Expense

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



	Three Months Ended September 30,				Nine Months Ended September 30,
	2018		2017		2018		2017
Cost of revenue	$	1,268	$	1,113	$	3,706	$	3,224
Research and development	12,010		11,048		34,700		30,977
Sales and marketing	6,537		5,115		18,771		12,651
General and administrative	3,439		2,876		9,406		8,139
Total stock-based compensation	$	23,254	$	20,152	$	66,583	$	54,991

As of September 30, 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).



	September 30, 2018
	Stock Option		RSU		ESPP		Restricted Stock
Unrecognized stock-based compensation expense	$	60,754	$	161,733	$	9,722	$	5,825
Weighted-average amortization period	3.6 years		3.2 years		1.2 years		4.0 years

Restricted Stock

Pursuant to the close of an acquisition during the current quarter (see Note 2), we issued 21,749 restricted shares of our common stock to certain key employees. These restricted shares vest over 4 years from the acquisition date and any unvested shares will be forfeited upon termination of such employees under certain conditions. The acquisition date fair value of these shares will be recognized as stock-based compensation over their vesting period.