STOCK-BASED COMPENSATION (Tables)

12 Months Ended

Dec. 31, 2020

Summary of Stock Option Activity

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

	For the Year Ended December 31,
	2020					2019					2018
				Weighted					Weighted					Weighted
				Average					Average					Average
				Exercise					Exercise					Exercise
	Shares			Price		Shares			Price		Shares			Price
Grants outstanding at beginning of    the period		8,346,348		$	61.01		8,391,171		$	46.09		8,806,204		$	29.74
Granted		1,402,409			122.25		1,429,652			132.97		2,152,439			90.15
Exercised		(1,445,054	)		53.21		(1,055,715	)		30.73		(2,119,306	)		22.89
Cancelled and forfeited		(458,756	)		108.60		(418,760	)		84.15		(448,166	)		46.11
Grants outstanding at end of the period		7,844,947		$	70.61		8,346,348		$	61.01		8,391,171		$	46.09
Grants exercisable at end of the period		2,020,322		$	70.16		2,368,621		$	45.33		2,304,791		$	27.69
Grants vested and expected to vest at    end of the period		7,500,726		$	68.24		7,987,427		$	58.65		6,643,835		$	45.43

     The weighted-average grant date fair value per share of stock options granted during the years ended December 31, 2020, 2019 and 2018 was $61.38, $70.93 and $44.66, respectively. 

			Weighted
	Aggregate		Average
	Intrinsic		Remaining
	Value		Contractual
	(in thousands)		Life (Years)
Options outstanding at December 31, 2020	$	783,558		7.0
Options exercisable at December 31, 2020	$	202,709		6.2
Options vested and expected to vest at December 31, 2020	$	766,936	6.9

Summary of Company's Stock Options Vested and Exercised

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

	For the Year Ended December 31,
	2020		2019		2018
	(in thousands)
Aggregate grant date fair value of stock options    vested	$	80,355	$	50,878	$	16,316
Aggregate intrinsic value of stock options    exercised	$	144,750	$	109,707	$	158,936

Summary of Employee Stock Purchase Plan Activity and Expense

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

	For the Year Ended December 31,
	2020		2019		2018
Number of shares purchased		102,031		92,086		75,094
Proceeds received (in millions)	$	7.5	$	5.1	$	2.3

Summary of Stock-Based Compensation Expense by Function Included within Consolidated Statements of Operations and Comprehensive Loss

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

	For the Year Ended December 31,
	2020		2019		2018
	(in thousands)
Research and development	$	41,671	$	27,681	$	14,214
Selling, general and administrative		66,399		50,921		35,913
Total stock-based compensation	$	108,070	$	78,602	$	50,127

Summary of Stock-Based Compensation Expense by Grant Type Included within 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,
	2020		2019		2018
	(in thousands)
Stock options	$	68,832	$	53,427	$	37,671
Restricted stock awards/units		33,457		20,103		10,632
Employee stock purchase plan		5,781		5,072		1,824
Total stock-based compensation	$	108,070	$	78,602	$	50,127

Restricted Stock Awards (RSAs) [Member]

Summary of Restricted Stock Award and Restricted Stock Units Activity

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

	For the Year Ended December 31,
	2020					2019					2018
				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		136,125		$	42.98		252,321		$	42.37		411,781		$	37.23
Granted		—			—		—			—		27,590			98.57
Vested		(83,750	)		34.65		(100,840	)		40.54		(187,050	)		39.34
Forfeited		(3,500	)		142.71		(15,356	)		48.94		—			—
Grants outstanding at end of the period		48,875		$	50.11		136,125		$	42.98		252,321		$	42.37

Restricted Stock Units (RSUs) [Member]

Summary of Restricted Stock Award and Restricted Stock Units Activity

The following table summarizes the Company’s RSU activity for the periods indicated:

	For the Year Ended December 31
	2020					2019					2018
				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		605,874		$	121.61		251,298		$	81.21		66,552		$	33.72
Granted		623,552			120.43		511,283			131.18		230,736			87.95
Vested		(162,970	)		120.28		(84,068	)		75.12		(30,276	)		33.23
Forfeited		(119,377	)		118.48		(72,639	)		103.03		(15,714	)		71.45
Grants outstanding at end of the period		947,079		$	121.46		605,874		$	121.61		251,298		$	81.21

Stock Options [Member]

Assumptions for Measuring Fair Values of Stocks

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: 

	For the Year Ended December 31,
	2020	2019	2018
Risk-free interest rate (1)	0.1 - 1.3%	1.4 - 2.5%	2.5 - 3.0%
Expected dividend yield (2)	—	—	—
Expected term (3)	5.00 years	5.04 years	5.06 years
Expected volatility (4)	57.3 - 68.2%	52.5 - 68.9%	52.4 - 60.8%

(1) The risk-free interest rate is estimated using an average of Treasury bill interest rates over a historical period commensurate with the expected term of the option that correlates to the prevailing interest rates at the time of grant.

(2) The expected dividend yield is zero as the Company has not paid any dividends to date and does not expect to pay dividends in the future.

(3) The expected term is estimated using historical exercise behavior.

(4) The expected volatility is the implied volatility in exchange-traded options of the Company’s common stock.