Stockholders' equity (Tables)

9 Months Ended

Sep. 30, 2017

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, 2016		1,378,352		$	9.38		7.81	$	9,829
Options granted		796,790			16.92
Options exercised		(41,780	)		9.35
Options forfeited		(59,820	)		12.42
Outstanding at September 30, 2017		2,073,542		$	12.19		7.99	$	6,280
Exercisable as of September 30, 2017		837,318		$	9.39		6.43	$	4,021

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 SEPTEMBER 30,						NINE MONTHS ENDED SEPTEMBER 30,
	2017			2016			2017			2016
Expected volatility		91	%		87	%		91	%		81	%
Expected dividends		0	%		0	%		0	%		0	%
Expected terms (years)		6.00			6.00			6.00			6.00
Risk free rate		1.87	%		1.07	%		1.89	%		1.23	%
Weighted average grant-date fair value of stock options	$	11.13		$	6.50		$	12.69		$	5.76