Warrants - Issued and Outstanding (Details) - USD ($) $ / shares in Units, $ in Thousands	Dec. 31, 2018	Oct. 16, 2018	Dec. 31, 2017	Mar. 01, 2017	Dec. 22, 2016	Nov. 09, 2015	Oct. 20, 2015	Nov. 26, 2014	Jul. 19, 2013	Jul. 01, 2012
Class of Warrant or Right [Line Items]
Number of Shares Underlying Warrants (in shares)			280,876
Common Stock
Class of Warrant or Right [Line Items]
Number of Shares Underlying Warrants (in shares)	118,122	160,657	124,326
Common Stock | Warrants Issued 7/19/2013
Class of Warrant or Right [Line Items]
Number of Shares Underlying Warrants (in shares)	32,983		101,010						101,010
Price per Share (in dollars per share)	$ 9.1		$ 3.96						$ 3.96
Fair Value	$ 122		$ 244
Common Stock | Warrants Issued 11/26/2014
Class of Warrant or Right [Line Items]
Number of Shares Underlying Warrants (in shares)	6,680		21,928					21,928
Price per Share (in dollars per share)	$ 16.47		$ 3.42					$ 3.42
Fair Value	$ 49		$ 47
Common Stock | Warrants Issued 3/1/2017
Class of Warrant or Right [Line Items]
Number of Shares Underlying Warrants (in shares)	1,388		1,388	1,388
Price per Share (in dollars per share)	$ 5.94		$ 5.94	$ 5.94
Fair Value	$ 5		$ 5
Common Stock | Warrants Issued 10/20/2015
Class of Warrant or Right [Line Items]
Number of Shares Underlying Warrants (in shares)	41,650
Price per Share (in dollars per share)	$ 16.47
Fair Value	$ 396
Common Stock | Warrants Issued 11/9/2015
Class of Warrant or Right [Line Items]
Number of Shares Underlying Warrants (in shares)	25,709
Price per Share (in dollars per share)	$ 16.47
Fair Value	$ 244
Common Stock | Warrants Issued 12/22/2016
Class of Warrant or Right [Line Items]
Number of Shares Underlying Warrants (in shares)	9,712
Price per Share (in dollars per share)	$ 10.03
Fair Value	$ 45
Common Stock | Common Stock Warrants
Class of Warrant or Right [Line Items]
Number of Shares Underlying Warrants (in shares)	118,122		124,326
Redeemable Convertible Preferred Stock
Class of Warrant or Right [Line Items]
Number of Shares Underlying Warrants (in shares)	0		156,550
Redeemable Convertible Preferred Stock | Preferred Stock Warrants
Class of Warrant or Right [Line Items]
Number of Shares Underlying Warrants (in shares)			156,550
Warrants exercise period from date of issuance			10 years
Series 5 Preferred Stock | Warrants Issued 7/1/2012
Class of Warrant or Right [Line Items]
Number of Shares Underlying Warrants (in shares)			54,917							54,917
Price per Share (in dollars per share)			$ 9.1							$ 9.10
Fair Value			$ 255
Series 5 Preferred Stock | Warrants Issued 7/19/2013
Class of Warrant or Right [Line Items]
Number of Shares Underlying Warrants (in shares)			21,989						32,983
Price per Share (in dollars per share)			$ 9.1						$ 9.10
Fair Value			$ 122
Series 6 Preferred Stock | Warrants Issued 11/26/2014
Class of Warrant or Right [Line Items]
Number of Shares Underlying Warrants (in shares)			6,310					6,310
Price per Share (in dollars per share)			$ 16.47					$ 16.47
Fair Value			$ 49
Series 6 Preferred Stock | Warrants Issued 10/20/2015
Class of Warrant or Right [Line Items]
Number of Shares Underlying Warrants (in shares)			39,339				39,339
Price per Share (in dollars per share)			$ 16.47				$ 16.47
Fair Value			$ 396
Series 6 Preferred Stock | Warrants Issued 11/9/2015
Class of Warrant or Right [Line Items]
Number of Shares Underlying Warrants (in shares)			24,283			24,283
Price per Share (in dollars per share)			$ 16.47			$ 16.47
Fair Value			$ 244
Series 7 Preferred Stock | Warrants Issued 12/22/2016
Class of Warrant or Right [Line Items]
Number of Shares Underlying Warrants (in shares)			9,712		9,712
Price per Share (in dollars per share)			$ 10.03		$ 10.03
Fair Value			$ 45