Nature of Business - Liquidity - Financial Statement Items (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Nature of Business
Net loss	$ (20,410)	$ (24,092)
Accumulated deficit	381,896	362,316
Cash and cash equivalents	$ 16,970	$ 4,024	$ 21,450