Income Taxes (Provision for Income Taxes) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Income tax benefit at statutory federal rate	$ (39,033)	$ (26,583)	$ (14,250)
Research and development credits	(2,691)	(1,240)	1,128
Foreign rate differential	1,249	0	0
Expired tax attributes	2,228	(5)	31
Impact of the 2017 Tax Act	71,199	0	0
Stock-based compensation	2,253	0	0
Change in valuation allowance	(35,246)	25,091	12,042
Other permanent differences	41	2,737	1,049
Provision for income taxes	$ 0	$ 0	$ 0