Income Taxes (Provision for Income Taxes) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Federal statutory rate	$ (41,888)	$ (39,033)	$ (26,583)
State tax benefit	(14,449)	0	0
Research and development credits	(5,164)	(2,691)	(1,240)
Foreign rate differential	(58)	1,249	0
Change in valuation allowance	55,167	(35,246)	25,091
Stock-based compensation	4,041	2,253	0
Expired tax attributes	1,936	2,228	(5)
Impact of the 2017 Tax Act	0	71,199	0
Other permanent differences	415	41	2,737
Provision for income taxes	$ 0	$ 0	$ 0