Stockholders' equity (Tables)

3 Months Ended

Mar. 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		787,490			16.55
Options exercised		(19,731	)		10.30
Options forfeited		(60,849	)		13.42
Outstanding at March 31, 2018		2,776,077		$	13.41		8.19	$	5,876
Exercisable as of March 31, 2018		1,115,975		$	10.67		6.63	$	4,321

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 MARCH 31,
	2018			2017
Expected volatility		75	%		91	%
Expected dividends		0	%		0	%
Expected terms (years)		6.00			6.00
Risk free rate		2.72	%		1.90	%
Weighted average grant-date fair value of stock options	$	11.11		$	13.02