Employee Benefit Plans (Tables)

12 Months Ended

Dec. 31, 2019

Employee Benefit Plans

Assumptions made for the purpose of estimating fair value

	2019	2018	2017
Expected term of options (years)	6	6	6
Risk free interest rate	1.52% - 2.53%	2.81% - 2.88%	1.89% - 2.16%
Volatility	69.32% - 87.94%	98.31% - 98.89%	99.24% - 102.16%

Schedule of Share-based Compensation, Stock Options, Activity

A summary of stock option activity for the year December 31, 2019 is as follows (in thousands except share amounts):

				Weighted
		Weighted		Average
		Average		Remaining	Aggregate
		Exercise		Contractual	Intrinsic
	Shares	Price		Terms	Value
Options outstanding at December 31, 2018	21,957,150	$	2.51	7.1	$	1,432
Granted	3,221,892		2.40	—		—
Exercised	(1,151,307)		1.56	—		—
Forfeited	(934,195)		3.50	—		—
Expired	(79,950)		9.43	—		—
Options outstanding at December 31, 2019	23,013,590	$	2.48	6.6	$	22,277
Options exercisable at December 31, 2019	16,608,362		2.57	5.8		16,285
Options unvested at December 31, 2019	6,405,228	$	2.22	8.9	$	5,992

Nonvested Restricted Stock Shares Activity

A summary of restricted stock activity for the year ended December 31, 2019 is as follows (in thousands except share amounts):

		Aggregate
		Intrinsic
	Shares	Value
Unvested restricted stock at December 31, 2018	2,347,347	$	—
Granted	3,201,892		—
Vested	(920,679)		—
Forfeited	(20,000)
Unvested restricted stock at December 31, 2019	4,608,560	$	14,563