In June 2011, the Company’s stockholders approved the 2011 Equity Incentive Plan (“2011 Plan”). The 2011 Plan, which as amended authorized 16.0 million shares of common stock to be issued, allowed for the grant of stock options, stock appreciation rights (“SARs”), restricted stock awards (“RSAs”), restricted stock units (“RSUs”), performance shares and performance units. During 2018, the 2011 Plan was merged into the 2018 Plan (defined below). As a result, there were no shares of common stock remaining available for future grant under the 2011 Plan.

In June 2013, the Company’s stockholders approved the 2013 Employee Stock Purchase Plan (“ESPP”) with approximately 0.3 million shares of common stock available to be issued. In June 2016 and 2019, the Company’s stockholders approved additional approximately 0.3 million and 0.5 million shares, respectively, of common stock available to be issued to be added to the 2013 ESPP. As of December 31, 2019, 0.6 million shares of common stock remain available for future grant under the 2013 ESPP.

In September 2014, the Company initiated the 2014 Employment Commencement Incentive Plan (“2014 Plan”) with approximately 0.6 million shares of common stock available to be issued. In October 2015, June 2017 and July 2018, the 2014 Plan was increased by 1.0 million, 3.8 million and 1.2 million shares of common stock available to be issued, respectively. As of December 31, 2019, 0.6 million shares of common stock remain available for future grant under the 2014 Plan.

In June 2018, the Company’s stockholders approved the 2018 Equity Incentive Plan (“2018 Plan”). The 2018 Plan, which authorized 2.9 million shares of common stock to be issued, allows for the grant of stock options, SARs, RSAs, RSUs, performance shares and performance units. The 2011 Plan was merged into the 2018 Plan and, as a result, all remaining shares in the 2011 Plan were transferred into the 2018 Plan. As of December 31, 2019, 3.4 million shares of common stock remain available for future grant under the 2018 Plan.

In general, stock options have a ten-year term and vest over a four-year period, with one-fourth of the underlying shares vesting on the first anniversary of the grant and 1/48th of the underlying shares vesting monthly thereafter, such that the underlying shares will be fully vested on the fourth anniversary of the grant, subject to the terms of the applicable plan under which they were granted.

The fair values of stock options granted during the periods presented are measured on the date of grant using the Black-Scholes-Merton option-pricing model, with the following assumptions:

The amounts estimated according to the Black-Scholes-Merton option-pricing model may not be indicative of the actual values realized upon the exercise of these options by the holders.

The following tables summarize the Company’s stock option activity for each of the periods indicated:

	For the Year Ended December 31,
	2019					2018					2017
				Weighted					Weighted						Weighted
				Average					Average						Average
				Exercise					Exercise						Exercise
	Shares			Price		Shares			Price		Shares				Price
Grants outstanding at beginning of the period		8,391,171		$	46.09		8,806,204		$	29.74		5,436,951			$	22.70
Granted		1,429,652			132.97		2,152,439			90.15		4,805,722		(1)		35.09
Exercised		(1,055,715	)		30.73		(2,119,306	)		22.89		(792,845	)			17.40
Expired and forfeited		(418,760	)		84.15		(448,166	)		46.11		(643,624	)			25.44
Grants outstanding at end of the period		8,346,348		$	61.01		8,391,171		$	46.09		8,806,204			$	29.74

Grants exercisable at end of the period		2,368,621		$	45.33		2,304,791		$	27.69		3,288,712			$	24.76
Grants vested and expected to vest at end of the period		7,987,427		$	58.65		6,643,835		$	45.43		6,910,022			$	28.49

The weighted-average grant date fair value per share of stock options granted during the years ended December 31, 2019, 2018 and 2017 was $70.93, $44.66 and $14.78, respectively.

			Weighted
	Aggregate		Average
	Intrinsic		Remaining
	Value		Contractual
	(in thousands)		Life (Years)
Options outstanding at December 31, 2019	$	587,191	7.5
Options exercisable at December 31, 2019	$	199,438		6.0
Options vested and expected to vest at December 31, 2019	$	578,938	7.4

The following table summarizes the Company’s stock options vested and exercised for each of the periods indicated:

For the years ended December 31, 2019, 2018 and 2017, the Company has recognized approximately $0.1 million, $0.2 million and $0.9 million in stock-based compensation expense related to the options with performance-based criteria, respectively.

The Company grants RSAs to members of its board of directors and certain employees. The following table summarizes the Company’s RSA activity for each of the periods indicated:

	For the Year Ended December 31,
	2019					2018					2017
				Weighted					Weighted					Weighted
				Average					Average					Average
				Grant Date					Grant Date					Grant Date
	Shares			Fair Value		Shares			Fair Value		Shares			Fair Value
Grants outstanding at beginning of the period		252,321		$	42.37		411,781		$	37.23		153,170		$	34.53
Granted		—			—		27,590			98.57		341,500			34.58
Vested		(100,840	)		40.54		(187,050	)		39.34		(63,264	)		14.60
Cancelled		(15,356	)		48.94		—			—		(19,625	)		43.02
Grants outstanding at end of the period		136,125		$	42.98		252,321		$	42.37		411,781		$	37.23

In September 2016, the Company granted certain executives RSAs with a performance conditions relating to certain sales targets. If the sales targets are achieved within the required time frame, the number of RSAs may be increased from 71,925 to 89,906 shares. In December 2017, the Company modified the expiration date of these RSAs from June 30, 2018 to January 1, 2019. As a result of this modification, the fair value per RSA was changed from $48.94 to $54.29. Through December 31, 2017, the Company had not recorded any stock-based compensation expense associated with these awards as the achievement of the performance conditions was not deemed probable. As of December 31, 2018, the first sales target related to these RSAs was achieved and, accordingly, the Company recognized approximately $3.3 million of stock-based compensation expense during the year ended December 31, 2018. The second target was not achieved and the shares related to this target were subsequently cancelled and no expense was recognized.

The Company also grants RSUs to members of its board of directors and employees. The following table summarizes the Company’s RSU activity for the periods indicated:

	For the Year Ended December 31,
	2019					2018					2017
				Weighted					Weighted					Weighted
				Average					Average					Average
				Grant Date					Grant Date					Grant Date
	Shares			Fair Value		Shares			Fair Value		Shares			Fair Value
Grants outstanding at beginning of the period		251,298		$	81.21		66,552		$	33.72		—		$	—
Granted		511,283			131.18		230,736			87.95		181,029			33.03
Vested		(84,068	)		75.12		(30,276	)		33.23		(78,017	)		32.63
Cancelled		(72,639	)		103.03		(15,714	)		71.45		(36,460	)		32.63
Grants outstanding at end of the period		605,874		$	121.61		251,298		$	81.21		66,552		$	33.72

In March 2017, the Company granted certain executives 156,029 RSUs with performance conditions relating to certain sales target and regulatory milestones, which were achieved between June 2017 and March 2019. As of December 31, 2019, there were no RSUs with performance conditions remaining to be vested. For the years ended December 31, 2019, 2018 and 2017, the Company recognized approximately $0.5 million, $0.2 million and $2.9 million of stock-based compensation expense, respectively.

The Company issues SARs on the same terms as options granted to employees. The grant date fair value of the SARs is determined using the same valuation assumptions as for stock options described above. Stock-based compensation expense is recognized on a straight-line basis over the vesting period of the SARs.

The following table summarizes the Company’s SAR activity for each of the periods indicated:

	For the Year Ended December 31,
	2019						2018				2017
				Weighted					Weighted				Weighted
				Average					Average				Average
				Exercise					Exercise				Exercise
	Shares			Price			Shares		Price		Shares		Price
Grants outstanding at beginning of the period		100,000		$	23.85			100,000	$	23.85		100,000	$	23.85
Exercised		(100,000	)	$	(23.85	)		—	$	—		—	$	—
Grants outstanding at end of the period		—		$	—			100,000	$	23.85		100,000	$	23.85
Grants exercisable at end of the period		—		$	—			100,000	$	23.85		100,000	$	23.85
Grants vested and expected to vest at end of the period		—		$	—			100,000	$	23.85		100,000	$	23.85

Under the Company’s 2013 ESPP, participating employees purchase common stock through payroll deductions. The purchase price is equal to 85% of the lower of the closing price of the Company’s common stock on the first business day and the last business day of the relevant purchase period. The 24-month offering period will end between February 29, 2020 and August 31, 2021. The following table summarizes the Company’s ESPP activity for each of the periods indicated:

For the years ended December 31, 2019, 2018 and 2017, total stock-based compensation expense was $78.6 million, $50.1 million and $30.5 million, respectively. Included in the amounts for the year ended December 31, 2017 is $2.1 million of stock-based compensation expense incurred in connection with the resignation of the Company’s former CEO.

The following table summarizes stock-based compensation expense by function included within the consolidated statements of operations and comprehensive loss:

The following table summarizes stock-based compensation expense by grant type included within the consolidated statements of operations and comprehensive loss:

	For the Year Ended December 31,
	2019		2018		2017
	(in thousands)
Stock options	$	53,427	$	37,671	$	23,416
Restricted stock awards/units		20,103		10,632		5,295
Employee stock purchase plan		5,072		1,824		1,754
Total stock-based compensation	$	78,602	$	50,127	$	30,465

As of December 31, 2019, there was $181.1 million of total unrecognized stock-based compensation expense related to the Company’s stock-based compensation plans. The expense is expected to be recognized over a weighted-average period of approximately 3 years. Of this amount, $109.5 million relates to options with service conditions only, $22.1 million relates to awards with service and market conditions, less than $0.1 million relates to awards with performance conditions, and the remaining $49.5 million related to restricted stock awards or restricted stock units with service conditions only.