Income Taxes - Operating Loss Carryforwards (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Operating Loss Carryforwards
Minimum ownership change allowed to avoid a limitation of use of net operating loss carryforwards (as a percent)	50.00%
Provision for income taxes	$ 20	$ (157)
Federal
Operating Loss Carryforwards
Operating loss carryforward	106,878
Research tax credits	2,811	1,799
State
Operating Loss Carryforwards
Operating loss carryforward	87,868
Research tax credits	382	$ 231
Foreign
Operating Loss Carryforwards
Operating loss carryforward	69,941
Indefinite federal net operating loss carryforward	$ 25,569