Income Taxes - Reconciliation of 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]
Balance as of beginning of period	$ 12,596	$ 12,004	$ 11,311
Additions for prior year tax positions	0	0	0
Reductions for prior year tax positions	(1,849)	0	0
Additions for current year tax positions	0	592	693
Reductions for current year tax positions	0	0	0
Reductions related to closed years/settlements with tax authorities	(2,269)	0	0
Balance as of end of period	$ 8,478	$ 12,596	$ 12,004