Income Taxes - Operating Loss Carryforwards (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
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	$ (157)
Federal
Operating Loss Carryforwards
Operating loss carryforward	81,614
Research tax credits	1,799	$ 1,308
State
Operating Loss Carryforwards
Operating loss carryforward	63,520
Research tax credits	231	$ 119
Foreign
Operating Loss Carryforwards
Operating loss carryforward	$ 69,970