Income Taxes - Components (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Current:
U.S. federal	$ 0	$ 1	$ 123
State	10	26	26
Total current	10	27	149
Deferred:
U.S. Federal	(125)	0	0
State	0	0	0
Total deferred	(125)	0	0
Provision for income taxes	$ (115)	$ 27	$ 149