Stockholders' equity (Tables)

3 Months Ended

Mar. 31, 2017

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
Outstanding at December 31, 2016		1,378,352		$	9.38		7.81	$	9,829
Options granted		696,100			17.35
Options forfeited		(59,063	)		12.37
Outstanding at March 31, 2017		2,015,389		$	12.04		8.34	$	9,806
Exercisable as of March 31, 2017		713,412		$	9.26		6.44	$	5,299

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,
	2017			2016
Expected volatility		91	%		80	%
Expected dividends		0	%		0	%
Expected terms (years)		6.00			6.00
Risk free rate		1.90	%		1.27	%
Weighted average grant-date fair value of stock options	$	13.02		$	5.67