Consolidated Statements of Changes in Redeemable Convertible Preferred Stock - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Beginning balance	$ 97,963	$ 86,662	$ 75,361
Accretion of redeemable convertible preferred stock	6,258	11,301	11,301
Issuance of preferred stock upon exercise of preferred stock warrants	1,863
Conversion of preferred stock into common stock at initial public offering	(106,084)
Ending balance		97,963	86,662
Series 1
Beginning balance	$ 60,184	$ 48,883	$ 37,582
Beginning balance, shares	5,334	5,334	5,334
Accretion of redeemable convertible preferred stock	$ 6,258	$ 11,301	$ 11,301
Conversion of preferred stock into common stock at initial public offering	$ (66,442)
Conversion of preferred stock into common stock at initial public offering, shares	(5,334)
Ending balance		$ 60,184	$ 48,883
Ending balance, shares	0	5,334	5,334
Series 2
Beginning balance	$ 37,779	$ 37,779	$ 37,779
Beginning balance, shares	2,552	2,552	2,552
Issuance of preferred stock upon exercise of preferred stock warrants	$ 1,863
Issuance of preferred stock upon exercise of preferred stock warrants, shares	106
Conversion of preferred stock into common stock at initial public offering	$ (39,642)
Conversion of preferred stock into common stock at initial public offering, shares	(2,658)
Ending balance		$ 37,779	$ 37,779
Ending balance, shares	0	2,552	2,552