Income Taxes - Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Gross unrecognized tax benefit, beginning of period	$ 1,468	$ 1,468
Additions based on tax positions related to the current year	0	0
Decreases based on tax positions in a prior period	0	0
Gross unrecognized tax benefit, end of period	$ 1,468	$ 1,468