Convertible Stock and Warrants - Warrants Outstanding and Respective Fair Value (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Series F warrants
Class of Warrant or Right [Line Items]
Warrants outstanding (in shares)	581,182	581,182
Fair value of warrants	$ 2,327	$ 8,378
Series G warrants
Class of Warrant or Right [Line Items]
Warrants outstanding (in shares)	279,606	279,606
Fair value of warrants	$ 42	$ 1,447
Preferred stock warrants
Class of Warrant or Right [Line Items]
Warrants outstanding (in shares)	860,788	860,788
Fair value of warrants	$ 2,369	$ 9,825