Stockholders' equity (Tables)

6 Months Ended

Jun. 30, 2019

Federal Home Loan Banks [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, 2018		2,897,294		$	9.04		7.96	$	—
Options forfeited		(266,197	)		11.76
Outstanding at June 30, 2019		2,631,097		$	8.76		7.60	$	—
Exercisable as of June 30, 2019		1,160,663		$	11.58		6.01	$	—

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,
	2019		2018			2019		2018
Expected volatility		NA		86	%		NA		77	%
Expected dividends		NA		0	%		NA		0	%
Expected terms (years)		NA		6.00			NA		6.00
Risk free rate		NA		2.82	%		NA		2.74	%
Weighted average grant-date fair value of stock options		NA	$	9.60			NA	$	10.83