Income Taxes - Valuation allowances, uncertain tax positions and accrued interest and penalties (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Valuation allowance
Increase (decrease) in valuation allowance	$ 6,531	$ (7,445)	$ (11,219)
Interest and penalties
Accrued interest or penalties for income tax liabilities	0	51	0
Change in reserve for uncertain tax benefits
Reserve at beginning of year	662	560	560
Increases related to prior year tax positions	7	217	0
Lapse of statute of limitations	(562)	(115)
Total	107	$ 662	$ 560
Uncertain tax positions
Tax impact of reasonably possible change in uncertain tax positions	$ 107