Common stock (Tables)

12 Months Ended

Dec. 31, 2014

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, 2013		628,754		$	8.83		7.50
Options granted		229,082			10.13
Options exercised		(86	)		5.76
Options forfeited/expired		(33,146	)		5.98
Outstanding at December 31, 2014		824,604		$	6.79		7.16	$	788,840
Exercisable as of December 31, 2014		429,369		$	6.79		5.58	$	646,363

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:

	DECEMBER 31, 2014			DECEMBER 31, 2013			DECEMBER 31, 2012
Expected volatility		108	%		94	%		90	%
Expected dividends		0	%		0	%		0	%
Expected terms (years)		6.00			6.25			6.25
Risk free rate		1.67	%		2.11	%		1.79	%
Weighted average grant-date fair value of stock options	$	8.19		$	0.40		$	0.14