Schedule II - Valuation and qualifying accounts (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Allowance for Doubtful Accounts
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Year	$ (1,769)	$ (1,587)	$ (1,297)
Additions Charged to Costs and Expenses	(1,055)	(1,012)	(612)
Deduction	469	830	322
Balance at End of Year	(2,355)	(1,769)	(1,587)
Tax Valuation Allowance
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Year	(4,422)	(3,786)	(1,981)
Additions Charged to Costs and Expenses	(567)	(636)	(1,805)
Deduction	0	0	0
Balance at End of Year	$ (4,989)	$ (4,422)	$ (3,786)