Stockholders' equity (Tables)

12 Months Ended

Dec. 31, 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		2,056,640			9.09
Options exercised		(64,938	)		9.28
Options expired		(520	)		10.56
Options forfeited		(1,163,055	)		14.70
Outstanding at December 31, 2018		2,897,294		$	9.04		7.96	$	—
Exercisable as of December 31, 2018		1,202,066		$	11.03		6.13	$	—

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:

	YEARS ENDED DECEMBER 31,
	2018			2017			2016
Expected volatility		80	%		84	%		82	%
Expected dividends		0	%		0	%		0	%
Expected terms (years)		6.00			6.00			6.00
Risk free rate		2.80	%		1.90	%		1.24	%
Weighted average grant-date fair value of stock options	$	6.24		$	11.85		$	6.11