Income Taxes (Provision for Income Taxes) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Income tax benefit at statutory federal rate	$ (26,583)	$ (14,250)	$ (7,256)
Research and development credits	(1,240)	1,128	991
Expired tax attributes	(5)	31	(60)
Change in valuation allowance	25,091	12,042	6,322
Other	2,737	1,049	3
Provision for income taxes	$ 0	$ 0	$ 0