Statement of Changes in Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) shares in Thousands, $ in Thousands	Total	Convertible Preferred Stock	Common Stock	Additional Paid-in Capital	Accumulated Deficit
Beginning balance at Dec. 31, 2017	$ (192,652)		$ 2	$ 4,292	$ (196,946)
Beginning balance, shares at Dec. 31, 2017		304,958
Beginning balance at Dec. 31, 2017		$ 187,136
Beginning balance, shares at Dec. 31, 2017			231
Exercises of stock options	31		$ 1	30
Exercises of stock options, shares			20
Share-based compensation expense	144			144
Net loss	(5,497)				(5,497)
Ending balance at Mar. 31, 2018	(197,974)		$ 3	4,466	(202,443)
Ending balance, shares at Mar. 31, 2018		304,958
Ending balance at Mar. 31, 2018		$ 187,136
Ending balance, shares at Mar. 31, 2018			251
Beginning balance at Dec. 31, 2018	71,042		$ 177	291,908	(221,043)
Beginning balance, shares at Dec. 31, 2018			17,744
Exercises of stock options	1,407		$ 5	1,402
Exercises of stock options, shares			483
Share-based compensation expense	501			501
Net loss	(7,529)				(7,529)
Ending balance at Mar. 31, 2019	$ 65,421		$ 182	$ 293,811	$ (228,572)
Ending balance, shares at Mar. 31, 2019			18,227