Stockholders' (Deficit) Equity - Preferred Stock Warrants (Details) - USD ($) $ / shares in Units, $ in Thousands							1 Months Ended	3 Months Ended	6 Months Ended	12 Months Ended
	May 07, 2018	Feb. 08, 2018	Feb. 24, 2017	Aug. 07, 2015	Aug. 05, 2013	Nov. 16, 2012	Jun. 30, 2014	Jun. 30, 2018	Jun. 30, 2018	Dec. 31, 2016	Dec. 31, 2012	Feb. 07, 2018	Dec. 31, 2017	Feb. 28, 2017	Dec. 31, 2015	Jun. 27, 2014
Preferred Stock Warrants
Convertible preferred stock warrants reclassified to additional paid-in capital	$ 855							$ 855
Convertible preferred stock
Preferred Stock Warrants
Warrants Outstanding	630,372												396,795
Fair Value	$ 855												$ 157
Number of warrants converted	630,372
Warrants, weighted average exercise price	$ 1.46
Series C convertible preferred stock | Warrants Issued on August 5, 2013
Preferred Stock Warrants
Warrants issued					74,768
Term of warrants					10 years
Value of each warrant (in dollars per warrant)					$ 0.525
Exercise Price	$ 1.07				$ 1.07								$ 1.07
Warrants Outstanding	74,768												74,768
Initial Value					$ 39
Fair Value	$ 80												$ 33
Series C convertible preferred stock | Warrants Issued on November 16, 2012
Preferred Stock Warrants
Exercise Price	$ 1.07												$ 1.07
Warrants Outstanding	186,916												186,916
Initial Value						$ 96
Fair Value	$ 200												$ 82
Series C convertible preferred stock | Five Year Warrants Issued in November 2012
Preferred Stock Warrants
Warrants issued											278,506
Term of warrants											5 years
Value of each warrant (in dollars per warrant)											$ 0.34
Aggregate number of shares called by warrants										258,880
Proceeds from the exercise of preferred stock warrants										$ 277
Exercise Price											$ 1.07
Warrants Outstanding															258,880
Initial Value											$ 94
Series C convertible preferred stock | Ten Year Warrants Issued in November 2012
Preferred Stock Warrants
Warrants issued						186,916
Term of warrants						10 years
Value of each warrant (in dollars per warrant)						$ 0.515
Exercise Price						$ 1.07
Initial Value						$ 96
Series E convertible preferred stock
Preferred Stock Warrants
Warrants issued							76,334
Term of warrants							10 years
Value of each warrant (in dollars per warrant)							$ 1.11
Exercise Price							$ 2.62
Initial Value							$ 85
Series E convertible preferred stock | Warrants Issued on August 7, 2015
Preferred Stock Warrants
Warrants issued				29,580
Term of warrants				10 years
Value of each warrant (in dollars per warrant)				$ 1.13
Exercise Price	$ 2.62			$ 2.62									$ 2.62
Warrants Outstanding	29,580												29,580
Initial Value				$ 33
Fair Value	$ 41												$ 8
Series E convertible preferred stock | Warrants Issued on June 27, 2014
Preferred Stock Warrants
Exercise Price	$ 2.62												$ 2.62
Warrants Outstanding	76,334												76,334
Initial Value																$ 85
Fair Value	$ 174												$ 21
Series F convertible preferred stock
Preferred Stock Warrants
Aggregate number of shares called by warrants												233,577		29,197
Exercise Price								$ 1.37	$ 1.37			$ 1.37		$ 1.37
Adjusted warrants due to dilutive issuance									56,569
Series F convertible preferred stock | Warrants Issued on February 8, 2018
Preferred Stock Warrants
Warrants issued		233,577
Term of warrants		10 years
Value of each warrant (in dollars per warrant)		$ 0.44
Exercise Price	$ 1.37	$ 1.37
Warrants Outstanding	233,577
Initial Value		$ 103
Fair Value	$ 320
Series F convertible preferred stock | Warrants Issued on February 24, 2017
Preferred Stock Warrants
Warrants issued			29,197
Term of warrants			10 years
Value of each warrant (in dollars per warrant)			$ 0.13
Exercise Price	$ 1.37		$ 1.37										$ 1.37
Warrants Outstanding	29,197												29,197
Initial Value			$ 4
Fair Value	$ 40												$ 13
Common Stock
Preferred Stock Warrants
Aggregate number of shares called by warrants	100,558
Warrants, weighted average exercise price	$ 9.38