Stockholders' equity (Tables)

12 Months Ended

Dec. 31, 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 granted		4,016,500			2.91
Options exercised		(326,094	)		4.77
Options forfeited		(747,162	)		8.71
Outstanding at December 31, 2019		5,840,538		$	5.11		7.72	$	45,037
Exercisable as of December 31, 2019		1,805,043		$	9.85		3.36	$	7,264

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,
	2019			2018			2017
Expected volatility		90	%		80	%		84	%
Expected dividends		0	%		0	%		0	%
Expected terms (years)		6.07			6.00			6.00
Risk free rate		1.43	%		2.80	%		1.90	%
Weighted average grant-date fair value of stock options	$	2.17		$	6.24		$	11.85