Stockholders' equity (Tables)

9 Months Ended

Sep. 30, 2018

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 (IN THOUSANDS)
Outstanding at December 31, 2017		2,069,167		$	12.18		7.76	$	3,187
Options granted		2,054,540			9.10
Options exercised		(64,938	)		9.28
Options expired		(520	)		10.56
Options forfeited		(700,985	)		14.86
Outstanding at September 30, 2018		3,357,264		$	9.79		8.27	$	—
Exercisable as of September 30, 2018		1,248,644		$	11.03		6.39	$	—

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 SEPTEMBER 30,						NINE MONTHS ENDED SEPTEMBER 30,
	2018			2017			2018			2017
Expected volatility		83	%		91	%		80	%		91	%
Expected dividends		0	%		0	%		0	%		0	%
Expected terms (years)		6.00			6.00			6.00			6.00
Risk free rate		2.86	%		1.87	%		2.80	%		1.89	%
Weighted average grant-date fair value of stock options	$	2.20		$	11.13		$	6.25		$	12.69