Balance Sheet Account Detail - Prepaid Expenses and Other Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Prepaid insurance	$ 645	$ 721
Research and development supplies	1,883	1,458
Other miscellaneous prepaid operating expenses	3,389	1,751
Key employee life insurance – cash surrender value	4,150	0
Prepaid expenses and other assets	$ 10,067	$ 3,930