Income Taxes - Components of Income Tax Provision (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Current tax provision:
Federal	$ 0	$ 2	$ 0
State	0	0	0
Foreign	0	0	0
Total current provision	0	2	0
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
Federal	0	3	0
State	0	0	0
Foreign	0	0	0
Total deferred provision	0	3	0
Total tax provision	$ 0	$ 5	$ 0