STOCK-BASED COMPENSATION

3 Months Ended

Mar. 31, 2019

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

11. STOCK-BASED COMPENSATION

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

	For the Three Months Ended March 31,
	2019				2018
	Grants		Weighted Average Grant Date Fair Value		Grants		Weighted Average Grant Date Fair Value
Stock options		990,234	$	76.44		1,124,585	$	34.08
Restricted stock units		324,630	$	143.14		150,725	$	71.45
Restricted stock awards		—	$	—		17,090	$	71.45

Stock-based Compensation Expense

For the three months ended March 31, 2019 and 2018, total stock-based compensation expense was $16.1 million and $10.5 million, respectively. The increase in stock-based compensation expense for the three months was driven by increases in headcount and stock price as well as the achievement of a milestone related to the March 2017 restricted stock units with performance conditions. 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 March 31,
	2019		2018
	(in thousands)
Research and development	$	5,087	$	2,060
Selling, general and administrative		11,052		8,466
Total stock-based compensation expense	$	16,139	$	10,526

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 March 31,
	2019		2018
	(in thousands)
Stock options	$	11,457	$	8,973
Restricted stock awards/units		3,916		1,260
Employee stock purchase plan		766		293
Total stock-based compensation expense	$	16,139	$	10,526