Stockholders' Equity - Preferred Stock Warrants (Details) - USD ($) $ / shares in Units, $ in Thousands	Feb. 28, 2018	May 07, 2018	Feb. 08, 2018	Feb. 24, 2017	Aug. 07, 2015	Jun. 27, 2014	Aug. 05, 2013	Nov. 16, 2012
Convertible preferred stock outstanding
Preferred Stock Warrants
Warrants Outstanding (in shares)		630,372
Fair Value		$ 855
Series F convertible preferred stock
Preferred Stock Warrants
Exercise Price (in dollars per share)	$ 1.37
Warrants Issued on February 8, 2018 | Series F convertible preferred stock
Preferred Stock Warrants
Warrants issued (in shares)	233,577
Term of warrants	10 years
Value of each warrant (in dollars per share)	$ 0.44
Exercise Price (in dollars per share)	$ 1.37	$ 1.37
Warrants Outstanding (in shares)		233,577
Initial Value	$ 100		$ 103
Fair Value		$ 320
Warrants Issued on February 24, 2017 | Series F convertible preferred stock
Preferred Stock Warrants
Exercise Price (in dollars per share)		$ 1.37
Warrants Outstanding (in shares)		29,197
Initial Value				$ 4
Fair Value		$ 40
Warrants Issued on August 7, 2015 | Series E convertible preferred stock
Preferred Stock Warrants
Exercise Price (in dollars per share)		$ 2.62
Warrants Outstanding (in shares)		29,580
Initial Value					$ 33
Fair Value		$ 41
Warrants Issued on June 27, 2014 | Series E convertible preferred stock
Preferred Stock Warrants
Exercise Price (in dollars per share)		$ 2.62
Warrants Outstanding (in shares)		76,334
Initial Value						$ 85
Fair Value		$ 174
Warrants Issued on August 5, 2013 | Series C convertible preferred stock
Preferred Stock Warrants
Exercise Price (in dollars per share)		$ 1.07
Warrants Outstanding (in shares)		74,768
Initial Value							$ 39
Fair Value		$ 80
Warrants Issued on November 16, 2012 | Series C convertible preferred stock
Preferred Stock Warrants
Exercise Price (in dollars per share)		$ 1.07
Warrants Outstanding (in shares)		186,916
Initial Value								$ 96
Fair Value		$ 200