Stockholders' equity (Tables)

3 Months Ended

Mar. 31, 2015

Disclosure of Compensation Related Costs, Share-based Payments [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, 2014		824,604		$	6.79		7.16	$	789
Options granted		271,750			11.70
Options exercised		(28,020	)		2.57
Options forfeited		—			—
Options expired		(11,442	)		8.25
Outstanding at March 31, 2015		1,056,892		$	9.26		7.86	$	922
Exercisable as of March 31, 2015		436,129		$	7.23		5.88	$	767

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,
	2015			2014
Expected volatility		98	%		108	%
Expected dividends		0	%		0	%
Expected terms (years)		6.00			6.00
Risk free rate		1.49	%		1.52	%
Weighted average grant-date fair value of stock options	$	9.21		$	9.05