Equity Awards

3 Months Ended

Mar. 31, 2019

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

We currently have two equity incentive plans, our 2005 Stock Option Plan (the 2005 Plan) and our 2012 Equity Incentive Plan (the 2012 Plan). Our 2005 Plan was terminated in connection with our initial public offering in 2012 but continues to govern the terms of outstanding stock options that were granted prior to the termination of the 2005 Plan. We no longer grant equity awards pursuant to our 2005 Plan.

Our 2012 Plan provides for the grant of incentive stock options, nonqualified stock options, stock appreciation rights, RSUs, performance-based stock awards and other forms of equity compensation (collectively, equity awards). In addition, the 2012 Plan provides for the grant of performance cash awards. Incentive stock options may be granted only to employees. All other equity awards may be granted to employees, including officers, as well as directors and consultants. The share reserve may increase to the extent outstanding stock options under the 2005 Plan expire or terminate unexercised. Prior to January 2019, the share reserve also automatically increased on January 1 of each year, by up to 5% of the total number of shares of common stock outstanding on December 31 of the preceding year as determined by our board of directors. Our board of directors elected not to increase the number of shares of common stock reserved for issuance under the 2012 Plan pursuant to the provision described in the preceding sentence for the year ending December 31, 2019. In January 2019, our Board of Directors amended the 2012 Plan to remove the automatic increase provision. Therefore, for the remaining term of the 2012 Plan, the share reserve will not be increased without stockholder approval.

Our 2012 Employee Stock Purchase Plan (the 2012 ESPP) authorizes the issuance of shares of common stock pursuant to purchase rights granted to our employees. The price at which common stock is purchased under the 2012 ESPP is equal to 85% of the fair market value of our common stock on the first or last day of the offering period, whichever is lower. Offering periods are six months long and begin on February 1 and August 1 of each year. The number of shares of common stock reserved for issuance automatically increases on January 1 of each year until January 1, 2022, by up to 1% of the total number of shares of common stock outstanding on December 31 of the preceding year as determined by our board of directors. Our board of directors elected not to increase the number of shares of common stock reserved for issuance under the 2012 ESPP pursuant to the provision described in the preceding sentence for the year ending December 31, 2019.

Stock Options

Stock options are exercisable at a price equal to the market value of the underlying shares of common stock on the date of the grant as determined by our board of directors or, for those stock options issued subsequent to our initial public offering, the closing price of our common stock as reported on the New York Stock Exchange on the date of grant. Stock options granted under our 2005 Plan and the 2012 Plan to new employees generally vest 25% one year from the date the requisite service period begins and continue to vest monthly for each month of continued employment over the remaining three years. Options granted generally are exercisable for a period of up to ten years contingent on each holder’s continuous status as a service provider.

A summary of stock option activity for the three months ended March 31, 2019 was as follows:



	Number of Shares		Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Term (Years)	Aggregate Intrinsic Value (in thousands)
Outstanding at December 31, 2018	1,810,580		$	46.55
Exercised	(231,558	)	21.69			$	46,348
Outstanding at March 31, 2019	1,579,022		$	50.19	5.24	$	309,955
Vested and expected to vest as of March 31, 2019	1,574,667		$	50.13	5.23	$	309,205
Vested and exercisable as of March 31, 2019	1,147,276		$	37.62	4.29	$	239,629

Aggregate intrinsic value represents the difference between the estimated fair value of our common stock and the exercise price of outstanding, in-the-money options. The total fair value of stock options vested during the three months ended March 31, 2019 was $2.0 million.

As of March 31, 2019, total unrecognized compensation cost, adjusted for estimated forfeitures, related to unvested stock options was approximately $9.8 million. The weighted-average remaining vesting period of unvested stock options at March 31, 2019 was 2.09 years.

RSUs

A summary of RSU activity for the three months ended March 31, 2019 was as follows:



	Number of Shares		Weighted Average Grant Date Fair Value (Per Share)		Aggregate Intrinsic Value (in thousands)
Outstanding at December 31, 2018	10,201,660		$	121.84
Granted	4,257,565		233.78
Vested	(1,939,925	)	117.30		$	445,764
Forfeited	(357,319	)	121.26
Outstanding at March 31, 2019	12,161,981		$	160.28	$	2,997,807

RSUs outstanding as of March 31, 2019 comprise 11,167,127 RSUs with only a service condition and 994,854 RSUs with both a service condition and a performance condition. RSUs granted with only service vesting criteria under the 2012 Plan to employees generally vest over a four-year period. As of March 31, 2019, total unrecognized compensation cost, adjusted for estimated forfeitures, related to unvested RSUs was approximately $1.5 billion and the weighted-average remaining vesting period was 3.24 years.

Included in the RSU activity table above are shares granted to certain employees with both service and performance-based vesting criteria. These performance-based RSUs (PRSUs) are considered as eligible to vest when approved by the compensation committee of our board of directors in January of the year following the grant. The ultimate number of shares eligible to vest for PRSUs range from 0% to 180% of the target number of shares depending on achievement relative to the performance metric over the applicable period. The eligible shares subject to PRSUs granted during the three months ended March 31, 2019 will vest 33% in February 2020 and continue to vest quarterly for the remaining two subsequent years, contingent on each holder’s continuous status as a service provider on the applicable vesting dates. The number of PRSUs granted shown in the table above reflects the shares that could be eligible to vest at 100% of target for PRSUs and includes adjustments for over or under achievement for PRSUs granted in the prior year. We recognized $22.6 million and $15.6 million of stock-based compensation expense, net of actual and estimated forfeitures, associated with PRSUs on a graded vesting basis during the three months ended March 31, 2019 and 2018, respectively.