Consolidated Statements of Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	IPO	Series A Preferred Stock	Series A-1 Preferred Stock	Class A Common Stock	Common Stock	Common Stock IPO	Additional Paid-in Capital	Additional Paid-in Capital IPO	Accumulated Deficit	Accumulated Other Comprehensive Income (Loss)
Beginning Balance at Dec. 31, 2014	$ (5,327)		$ 3	$ 5	$ 1	$ 11		$ 62,203		$ (67,508)	$ (42)
Beginning Balance, shares at Dec. 31, 2014			3,129,084	5,225,879	1,164,105	11,237,257
Stock-based compensation	1,522							1,522
Stock-based compensation, shares						6,023
Repurchase of common stock	(1,500)							(1,500)
Repurchase of common stock, shares						(173,913)
Exercise of stock options	49							49
Exercise of stock options, shares						37,559
Other comprehensive income (loss)	29										29
Net loss	(10,824)									(10,824)
Ending Balance at Dec. 31, 2015	(16,051)		$ 3	$ 5	$ 1	$ 11		62,274		(78,332)	(13)
Ending Balance, shares at Dec. 31, 2015			3,129,084	5,225,879	1,164,105	11,106,926
Stock-based compensation	3,101							3,101
Issuance of common stock in offering, net of issuance costs		$ 66,103					$ 6		$ 66,097
Issuance of common stock in offering, net of issuance costs, shares							6,250,000
Conversion of convertible preferred stock and Class A common stock into common stock			$ (3)	$ (5)	$ (1)	$ 9
Conversion of convertible preferred stock and Class A common stock into common stock, shares			(3,129,084)	(5,225,879)	(1,164,105)	9,519,068
Cashless exercise of common stock warrant, shares						100,683
Exercise of stock options	$ 750					$ 1		749
Exercise of stock options, shares	163,968					163,968
Exercise of common stock warrant	$ 25							25
Exercise of common stock warrant, shares						10,029
Other comprehensive income (loss)	(290)										(290)
Net loss	(11,286)									(11,286)
Ending Balance at Dec. 31, 2016	42,352					$ 27		132,246		(89,618)	(303)
Ending Balance, shares at Dec. 31, 2016						27,150,674
Stock-based compensation	9,299							9,299
Issuance of common stock in offering, net of issuance costs	9,870					$ 1		9,869
Issuance of common stock in offering, net of issuance costs, shares						553,825
Purchase of convertible note capped call hedge	(12,922)							(12,922)
Equity component of convertible notes	22,094							22,094
Exercise of stock options	$ 2,869							2,869
Exercise of stock options, shares	497,175					497,175
Issuance of shares under employee stock purchase plan	$ 1,540							1,540
Issuance of shares under employee stock purchase plan, shares						128,786
Other comprehensive income (loss)	523										523
Net loss	(19,634)									(19,634)
Ending Balance at Dec. 31, 2017	$ 55,991					$ 28		$ 164,995		$ (109,252)	$ 220
Ending Balance, shares at Dec. 31, 2017						28,330,460