Summary of Significant Accounting Policies - Consolidated Statement of Cash Flows (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accounting Policies [Abstract]
Warrants exercised for common stock	$ 3,538	$ 0	$ 0
Non-cash information:	2,230	931	226
Shares exchanged between common and preferred stock	0	(38,389)	(3,150)
Additions to equipment in process included in accounts payable	0	100	0
Additions to equipment in process included in accounts payable	$ 606	$ 341	$ 18