Stockholders' equity (Tables)

12 Months Ended

Dec. 31, 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		837,540			16.63
Options exercised		(49,280	)		9.10
Options forfeited		(97,445	)		12.29
Outstanding at December 31, 2017		2,069,167		$	12.18	7.76	$3,187
Exercisable as of December 31, 2017		912,098		$	9.52	6.35	$2,248

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:

	YEARS ENDED DECEMBER 31,
	2017			2016			2015
Expected volatility		84	%		82	%		97	%
Expected dividends		0	%		0	%		0	%
Expected terms (years)		6.00			6.00			6.00
Risk free rate		1.90	%		1.24	%		1.48	%
Weighted average grant-date fair value of stock options	$	11.85		$	6.11		$	8.91