Stockholders' equity (Tables)

3 Months Ended

Mar. 31, 2020

Equity [Abstract]

Schedule of Stock Option Transactions and Number of Stock Options Outstanding

Stock option transactions and the number of stock options outstanding are summarized below:

	NUMBER OF SHARES			WEIGHTED AVERAGE EXERCISE PRICE		WEIGHTED AVERAGE REMAINING CONTRACTUAL LIFE (IN YEARS)		AGGREGATE INTRINSIC VALUE (IN THOUSANDS)
Outstanding at December 31, 2019		5,840,538		$	5.11		7.72	$	45,037
Options granted		290,700			7.63
Options exercised		(376,311	)		9.03
Options forfeited		(410,000	)		16.91
Outstanding at March 31, 2020		5,344,927		$	4.06		8.70	$	40,431
Exercisable as of March 31, 2020		1,040,512		$	7.28		5.56	$	5,462

Schedule of Restricted Stock Unit Transactions and Number of Unvested Stock Restricted Stock Units

Restricted stock unit transactions and the number of unvested stock restricted stock units are summarized below:

	NUMBER OF SHARES			WEIGHTED AVERAGE GRANT DATE FAIR VALUE
Non-vested at December 31, 2019		72,000		$	3.47
Restricted stock units granted		75,000			6.44
Restricted stock units vested		(13,000	)		3.47
Non-vested at March 31, 2020		134,000		$	5.13

Schedule of Weighted Average Assumptions

The fair value of stock options granted is estimated using the Black-Scholes option pricing model with the following weighted average assumptions:

	THREE MONTHS ENDED
	MARCH 31,
	2020			2019
Expected volatility		90	%	N/A
Expected dividends		0	%	N/A
Expected terms (years)	6.05			N/A
Risk free rate		0.78	%	N/A
Weighted average grant-date fair value of stock options	$	5.62		N/A