Income Taxes, Reconciliation of Federal Statutory Income Tax Rate to Effective Income Tax Rate (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Effective Tax Rate Reconciliation [Abstract]
United States federal statutory rate	21.00%	35.00%	35.00%
State taxes, net of federal benefit	(0.01%)	(0.01%)	(0.31%)
Valuation allowance	(24.33%)	(35.94%)	(35.43%)
Accounting method change - ASC 606	2.07%	0.00%	0.00%
R&D Credit	1.53%	1.43%	0.66%
Other	(0.27%)	(0.50%)	(0.39%)
Effective income tax rate	(0.01%)	(0.02%)	(0.47%)