Stockholders' equity (Tables)

12 Months Ended

Dec. 31, 2016

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
Outstanding at December 31, 2015		865,457		$	9.75		7.56	$	2,393
Options granted		658,710			8.77
Options exercised		(55,663	)		7.91
Options expired		(8,008	)		9.52
Options forfeited		(82,144	)		9.39
Outstanding at December 31, 2016		1,378,352		$	9.38		7.81	$	9,829
Exercisable as of December 31, 2016		575,422		$	9.30		6.20	$	4,152

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,
	2016			2015			2014
Expected volatility		82	%		97	%		108	%
Expected dividends		0	%		0	%		0	%
Expected terms (years)		6.00			6.00			6.00
Risk free rate		1.24	%		1.48	%		1.67	%
Weighted average grant-date fair value of stock options	$	6.11		$	8.91		$	8.19