STOCK-BASED COMPENSATION

6 Months Ended

Jun. 30, 2019

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

10. STOCK-BASED COMPENSATION

The following table summarizes the Company’s stock awards granted for each of the periods indicated:

	For the Three Months Ended June 30,								For the Six Months Ended June 30,
	2019				2018				2019				2018
	Grants		Weighted Average Grant Date Fair Value		Grants		Weighted Average Grant Date Fair Value		Grants		Weighted Average Grant Date Fair Value		Grants		Weighted Average Grant Date Fair Value
Stock options		155,265	$	67.80		528,096	$	45.45		1,145,499	$	75.27		1,652,681	$	37.71
Restricted stock units		81,000	$	121.34		18,695	$	95.38		405,630	$	138.79		169,420	$	74.09
Restricted stock awards		—	$	—		—	$	—		—	$	—		17,090	$	71.45

Stock-based Compensation Expense

For the three months ended June 30, 2019 and 2018, total stock-based compensation expense was $19.8 million and $15.3 million, respectively. For the six months ended June 30, 2019 and 2018, total stock-based compensation expense was $35.9 million and $25.8 million, respectively. The increase in stock-based compensation expense for the three and six months was primarily driven by increases in awards associated with growth in employees and the increased value of such awards. The following table summarizes stock-based compensation expense by function included within the unaudited condensed consolidated statements of operations and comprehensive loss:

	For the Three Months Ended June 30,				For the Six Months Ended June 30,
	2019		2018		2019		2018
	(in thousands)
Research and development	$	6,923	$	5,029	$	12,010	$	7,089
Selling, general and administrative		12,839		10,250		23,891		18,716
Total stock-based compensation expense	$	19,762	$	15,279	$	35,901	$	25,805

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

	For the Three Months Ended June 30,				For the Six Months Ended June 30,
	2019		2018		2019		2018
	(in thousands)
Stock options	$	13,777	$	9,382	$	25,234	$	18,355
Restricted stock awards/units		5,184		5,407		9,100		6,667
Employee stock purchase plan		801		490		1,567		783
Total stock-based compensation expense	$	19,762	$	15,279	$	35,901	$	25,805