Schedule II - Valuation and qualifying accounts (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Allowance for Doubtful Accounts
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Year	$ (1,587)	$ (1,297)	$ (893)
Additions Charged to Costs and Expenses	(1,012)	(612)	(607)
Deduction	830	322	203
Balance at End of Year	(1,769)	(1,587)	(1,297)
Tax Valuation Allowance
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Year	(3,786)	(1,981)	(1,603)
Additions Charged to Costs and Expenses	(636)	(1,805)	(378)
Deduction	0	0	0
Balance at End of Year	$ (4,422)	$ (3,786)	$ (1,981)