Common stock (Tables)

6 Months Ended

Jun. 30, 2014

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

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,
	2014		2013		2014		2013
Expected volatility		108%		93%		108%		93%
Expected dividends		0%		0%		0%		0%
Expected terms (years)		6.00		6.25		6.00		6.25
Risk free rate		1.70%		1.88%		1.67%		1.88%
Weighted average grant-date fair value of stock options		$8.30		$2.92		$8.42		$2.92