Share-Based Compensation

3 Months Ended

Mar. 31, 2020

Share-based Payment Arrangement [Abstract]

Non-Cash Share-Based Compensation Expense

Non-cash share-based compensation is measured at the grant date based on the fair value of the award and is recognized as expense over the requisite service period of the award (generally three to four years) using the straight-line method. Non-cash share-based compensation expense, consisting of expense for both stock options and RSUs, was classified in the condensed consolidated statements of operations and comprehensive loss as follows:


	Three Months Ended March 31,
	2020		2019
Research and development expenses	$	6,251	$	3,700
General and administrative expenses	10,628		3,630
	$	16,879	$	7,330

Stock Options

All stock option grants are awarded at fair value on the date of grant. The fair value of stock options is estimated using the Black-Scholes option pricing model and stock-based compensation is recognized on a straight-line basis over the requisite service period. Stock options granted generally become exercisable over a three-year or four-year period from the grant date. Stock options generally expire 10 years after the grant date.

The aggregate intrinsic value of stock options is calculated as the difference between the exercise price of the stock options and the fair value of the Company's common shares for those stock options that had exercise prices lower than the fair value of the Company's common shares at March 31, 2020.

As of March 31, 2020, unrecognized compensation expense related to unvested stock options totaled $77,324, which the Company expects to be recognized over a weighted-average period of 2.39 years. The Company expects approximately 3,821,719 of the unvested stock options to vest over the requisite service period.

The following table is a summary of the Company's stock option activity for the three months ended March 31, 2020:


	Number of Shares	Weighted Average Exercise Price	Weighted Average Remaining Contractual Term (in years)	Aggregate Intrinsic Value
Outstanding at December 31, 2019	9,423,015	$24.58
Granted	88,838	$45.63
Exercised	(350,461)	$7.44
Forfeited	(24,563)	$36.95
Outstanding at March 31, 2020	9,136,829	$25.41	7.51	$	119,446
Options exercisable at March 31, 2020	5,315,110	$16.47	6.65	$	106,407
Vested at March 31, 2020 and expected to vest in the future	9,136,829	$25.41	7.51	$	119,446

Restricted Share Units

The Company’s Restricted Share Units ("RSUs") are considered nonvested share awards and require no payment from the employee. For each RSU, employees receive one common share at the end of the vesting period. The employee can elect to receive the one common share net of taxes or pay for taxes separately and receive the entire share. Compensation cost is recorded based on the market price of the Company’s common stock on the grant date and is recognized on a straight-line basis over the requisite service period.

As of March 31, 2020, there was $24,379 of total unrecognized compensation cost related to Company RSUs that are expected to vest. These costs are expected to be recognized over a weighted-average period of 2.76 years. The total fair value of RSUs vested during three months ended March 31, 2020 was $4,829.

The following table is a summary of the RSU activity for the three months ended March 31, 2020:


	Number of Shares	Weighted Average Grant Date Fair Value
Unvested outstanding as of December 31, 2019	88,950	$57.40
Granted	482,175	$54.92
Forfeited	(4,750)	$54.28
Vested	(86,508)	$55.82
Unvested outstanding as of March 31, 2020	479,867	$55.22