In April 2015, the Company’s shareholders approved the 2015 Stock Option and Grant Plan, or the 2015 Plan, and in July 2016, the Company’s shareholders approved the 2016 Stock Option and Incentive Plan, or the 2016 Plan. In May 2018, the Company’s shareholders approved the 2018 Stock Option and Incentive Plan, or the 2018 Plan (collectively, the “Plans”). Subsequent to the IPO, no further options were granted under the 2015 Plan. The Plans provide for the issuance of equity awards in the form of restricted shares, options to purchase Common Shares which may constitute incentive stock options, or ISOs, or non-statutory stock options, or NSOs, unrestricted stock unit grants, and qualified performance and market-based awards to eligible employees, officers, directors, non-employee consultants and other key personnel. Terms of the equity awards, including vesting requirements, are determined by the Company’s board of directors, subject to the provisions of the Plans. Options granted by the Company typically vest over four years and have a contractual life of ten years. Restricted stock unit grants typically vest over two to three years.

The Company recognized stock-based compensation expense totaling $66.0 million, $49.5 million, and $39.3 million during the years ended December 31, 2020, 2019 and 2018, respectively. Stock‑based compensation expense by classification within the consolidated statements of operations and comprehensive income (loss) is as follows (in thousands):

	Years Ended December 31,
	2020		2019		2018
Research and development	$	35,120	$	23,273	$	17,557
General and administrative		30,898		20,784		17,428
Loss from equity method investment		—		5,467		4,275
Total	$	66,018	$	49,524	$	39,260

As of December 31, 2020, there was $132.5 million and $45.7 million of unrecognized compensation expense related to unvested stock options and restricted stock units, respectively, that is expected to be recognized over a weighted-average period of 2.8 and 2.1 years, respectively.

The fair value of each option issued to employees and non-employees was estimated at the date of grant using the Black-Scholes option pricing model with the following weighted-average assumptions:

	Years Ended December 31,
	2020			2019			2018
Options granted		2,182,773			2,832,784			2,209,597
Weighted-average exercise price	$	68.91		$	39.16		$	51.73
Weighted-average grant date fair value	$	42.28		$	24.57		$	33.82
Assumptions:
Expected volatility		69.2	%		68.9	%		71.9	%
Expected term (in years)		6.0			6.0			6.0
Risk-free interest rate		0.6	%		2.2	%		2.8	%
Expected dividend yield		0.0	%		0.0	%		0.0	%

The following table summarizes stock option activity under the Company’s equity award plans (intrinsic value in thousands):

	Shares			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Term (years)		Aggregate Intrinsic Value
Outstanding at December 31, 2019		7,782,437		$	31.30		8.2	$	231,554
Granted		2,182,773		$	68.91
Exercised		(1,517,735	)	$	23.53
Cancelled or forfeited		(345,495	)	$	41.61
Outstanding at December 31, 2020		8,101,980		$	42.44		7.8		896,666
Exercisable at December 31, 2020		3,838,372		$	30.43		7.0	$	470,907
Vested and expected to vest at December 31, 2020		8,101,980		$	42.44		7.8	$	896,666

During 2020 and 2019, the Company did not grant stock option awards subject to performance-based or market-based vesting conditions. As of December 31, 2020, options to purchase 940,646 Common Shares subject to performance-based vesting conditions were vested, as performance conditions were achieved, and there were 81,387 options to purchase Common Shares subject to performance-based vesting conditions outstanding.

During 2017, the Company granted 150,000 options with market-based vesting conditions, of which 75% vest at the end of a three-year service period and 25% vest at the end of a four-year service period. Upon achieving a specified average stock price in prior years, the market condition was satisfied. Expense for the options is being recognized over the requisite service period. As of December 31, 2020, 112,500 of the stock options had vested.

The total intrinsic value (the amount by which the fair market value exceeded the exercise price) of stock options exercised during the year ended December 31, 2020, 2019 and 2018 was $104.2 million, $42.2 million and $38.3 million, respectively.

The following table summarizes the restricted stock activity under the Company’s equity award plans:

	Shares			Weighted- Average Grant Date Fair Value
Unvested balance at December 31, 2019		699,534		$	56.53
Granted		437,670			77.92
Vested		(204,650	)		42.92
Cancelled or forfeited		(38,462	)		46.33
Unvested balance at December 31, 2020		894,092		$	70.55

During the years ended December 31, 2020, 2019 and 2018, the total fair value of restricted stock vested was $21.6 million, $3.6 million and $11.3 million, respectively.

During the years ended December 31, 2020, 2019 and 2018, the Company modified the terms of certain equity awards held by departing employees, resulting in $0.6 million, $0.1 million, and $3.8 million of stock-based compensation expense, respectively. During the year ended December 31, 2019, the Company modified the terms of certain equity awards held by non-employees. The modifications resulted in $2.9 million in stock-based compensation expense recorded during the period. For the year ended December 31, 2020 and 2018, there were no modification of options held by non-employees.

On July 19, 2016, the Company’s board of directors adopted its 2016 Employee Stock Purchase Plan, or the ESPP Plan, which was subsequently approved by its shareholders and became effective on October 19, 2016. The ESPP Plan authorizes the initial issuance of up to a total of 0.4 million shares of the Company’s common stock to participating employees. The Company activated its ESPP Plan on January 1, 2020. Activity to date has not been material.