Income Taxes - Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Beginning of year balance	$ 1,900	$ 1,900	$ 1,900
Decreases to tax positions taken during prior years	129	0	0
End of year balance	$ 1,771	$ 1,900	$ 1,900