Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2018

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Summary of Share Based Fair Value Assumptions

The fair value of options granted were estimated using the Black-Scholes-Merton option pricing model based on the assumptions in the table below:

	Years ended December 31,
	2018	2017	2016
Expected dividend yield	0%	0%	0%
Expected volatility	73% - 75%	70% - 72%	47%
Risk-free interest rate	2.35% - 3.02%	1.84% - 2.29%	1.13% - 2.09%
Expected life (in years)	5.5 - 6.1	5.5 - 6.3	5.5 - 6.3

Schedule of Stock Option Activity

The following table summarizes the Company’s stock option activity, including grants to non-employees, for the year ended December 31, 2018:

						Weighted-
						Average
				Weighted-		Remaining
	Number of			Average		Contractual
	Shares			Exercise Price		Term (Years)
Options outstanding at December 31, 2017		15,693,675		$	2.32		7.78
Granted		8,705,617			2.39
Forfeited		(925,495	)		1.59
Cancelled		(192,633	)		2.11
Exercised		(3,381,533	)		2.11
Options outstanding at December 31, 2018		19,899,631		$	2.42		7.82

Schedule of Stock Options Outstanding

The following table summarizes information about stock options outstanding at December 31, 2018:

					Weighted
					Average
			Weighted		Remaining
	Number of		Average		Contractual
	Shares		Exercise Price		Term (Years)
Exercisable at December 31, 2018		7,574,932	$	2.72		6.53
Vested or expected to vest at December 31, 2018		18,960,684	$	2.42		7.76