Stock-Based Compensation (Tables)

9 Months Ended

12 Months Ended

Sep. 30, 2019

Dec. 31, 2018

Share-based Payment Arrangement [Abstract]

Summary of Stock Options Activity

The following is a summary of stock options activity during the nine months ended September 30, 2019:

	Options Outstanding
	Number of Options			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Life (years)		Aggregate Intrinsic Value
Balance—January 1, 2019		1,867,747		$	9.01		9.64	$	19,573,295
Options granted		195,975		$	9.49
Options exercised		(487	)	$	4.14			$	1,630
Options cancelled		(165,622	)	$	8.76
Balance—September 30, 2019		1,897,613		$	9.10		9.02	$	498,451
Exercisable as of September 30, 2019		218,096		$	6.31		7.55	$	359,753
Vested and expected to vest as of September 30, 2019		1,954,263		$	9.13		8.75	$	498,451

The following is a summary of stock options activity during the year-ended December 31, 2018:

	Options Outstanding
	Number of Options			Weighted-Average Exercise Price		Weighted-Average Remaining Contractual Life (years)		Aggregate Intrinsic Value
Balance—January 1, 2018		437,400		$	6.01
Options granted		1,547,938		$	9.72
Options exercised		(4,465	)	$	4.13			$	14,410
Options cancelled		(113,126	)	$	7.02
Balance—December 31, 2018		1,867,747		$	9.01		9.64	$	19,573,295
Exercisable as of December 31, 2018		181,701		$	5.85		7.65	$	2,480,184
Vested and expected to vest as of December 31, 2018		1,867,747		$	9.01		9.64	$	19,573,295

Summary of Weighted Average Assumptions Used in Black-Scholes Option-Pricing Model to Determine Grant Fair Value

The following are weighted-average assumptions used in the Black-Scholes option-pricing model to determine grant fair value:

	September 30, 2018			September 30, 2019
	Weighted- Average			Weighted- Average
Expected term (in years)		0.00			5.77
Volatility		0.00	%		66.72	%
Risk-free interest rate		0.00	%		1.76	%
Dividend Yield		0.000			0.000

The following are weighted-average assumptions used in the Black-Scholes option-pricing model with to determine grant fair value:

	2017			2018
	Weighted- Average			Weighted- Average
Expected term (in years)		5.91			5.73
Volatility		53.51	%		55.06	%
Risk-free interest rate		2.16	%		2.59	%
Dividend Yield		0.000			0.000

Summary of Restricted Stock Activity

A summary of restricted stock activity within the Company’s equity plans and changes for the nine months ended September 30, 2019, is as follows:

Restricted Stock Awards	Shares			Weighted Average Grant- Date Fair Value
Nonvested at January 1, 2019		—		$	—
Granted		2,645,123		$	18.89
Vested		—		$	—
Forfeited		(69,141	)	$	19.50
Nonvested at September 30, 2019		2,575,982		$	18.87

Summary of Total Stock-based Compensation Expense by Function

The following table summarizes the total stock-based compensation expense by function for the three and nine months ended September 30, 2018 and 2019:

	Three Months Ended September 30,				Nine Months Ended September 30,
	2018		2019		2018		2019
	(in thousands)				(in thousands)
Sales and distribution expenses		5		1,617		11		2,533
Research and development expenses		8		1,327		21		1,878
General and administrative expenses		128		4,772		450		7,424
Total stock-based compensation expense	$	141	$	7,716	$	482	$	11,835