Stockholders' Equity - Preferred Stock Warrants (Details) - USD ($) $ / shares in Units, $ in Thousands			1 Months Ended	3 Months Ended
	May 07, 2018	Feb. 07, 2018	Feb. 28, 2017	Jun. 30, 2018	Dec. 31, 2017
Preferred Stock Warrants
Convertible preferred stock warrants reclassified to additional paid-in capital	$ 900			$ 900
Convertible preferred stock outstanding (in shares)
Preferred Stock Warrants
Warrants outstanding (in shares)	630,372				396,795
Fair Value	$ 855				$ 157
Number of warrants converted (in shares)	630,372
Warrants, weighted average exercise price (in dollars per share)	$ 1.46
Series F convertible preferred stock (in shares)
Preferred Stock Warrants
Period of warrants		10 years	10 years
Warrants fair value (in shares)		100,000	100,000
Warrant value (in dollars per share)		$ 0.44	$ 0.13
Exercise price (in dollars per share)		$ 1.37	$ 1.37
Series F convertible preferred stock (in shares) | Warrants Issued on February 8, 2018
Preferred Stock Warrants
Exercise price (in dollars per share)	$ 1.37
Warrants outstanding (in shares)	233,577
Initial Value	$ 103
Fair Value	$ 320
Series F convertible preferred stock (in shares) | Warrants Issued on February 24, 2017
Preferred Stock Warrants
Exercise price (in dollars per share)	$ 1.37				$ 1.37
Warrants outstanding (in shares)	29,197				29,197
Initial Value	$ 4				$ 4
Fair Value	$ 40				$ 13
Series E convertible preferred stock (in shares) | Warrants Issued on August 7, 2015
Preferred Stock Warrants
Exercise price (in dollars per share)	$ 2.62				$ 2.62
Warrants outstanding (in shares)	29,580				29,580
Initial Value	$ 33				$ 33
Fair Value	$ 41				$ 8
Series E convertible preferred stock (in shares) | Warrants Issued on June 27, 2014
Preferred Stock Warrants
Exercise price (in dollars per share)	$ 2.62				$ 2.62
Warrants outstanding (in shares)	76,334				76,334
Initial Value	$ 85				$ 85
Fair Value	$ 174				$ 21
Series C convertible preferred stock (in shares) | Warrants Issued on August 5, 2013
Preferred Stock Warrants
Exercise price (in dollars per share)	$ 1.07				$ 1.07
Warrants outstanding (in shares)	74,768				74,768
Initial Value	$ 39				$ 39
Fair Value	$ 80				$ 33
Series C convertible preferred stock (in shares) | Warrants Issued on November 16, 2012
Preferred Stock Warrants
Exercise price (in dollars per share)	$ 1.07				$ 1.07
Warrants outstanding (in shares)	186,916				186,916
Initial Value	$ 96				$ 96
Fair Value	$ 200				$ 82