Stockholders' equity (Tables)

6 Months Ended

Jun. 30, 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		287,750			11.42
Options exercised		(32,113	)		2.98
Options forfeited		—			—
Options expired		(12,442	)		8.34
Outstanding at June 30, 2015		1,067,799		$	9.24		7.58	$	425
Exercisable as of June 30, 2015		509,192		$	7.60		5.91	$	379

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 JUNE 30,						SIX MONTHS ENDED JUNE 30,
	2015			2014			2015			2014
Expected volatility		95	%		108	%		98	%		108	%
Expected dividends		0	%		0	%		0	%		0	%
Expected terms (years)		6.00			6.00			6.40			6.00
Risk free rate		1.25	%		1.70	%		1.47	%		1.67	%
Weighted average grant-date fair value of stock options	$	5.69		$	8.30		$	9.02		$	8.42