Schedule II - Valuation and qualifying accounts (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Allowance for Doubtful Accounts
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Year	$ (2,355)	$ (1,769)	$ (1,587)
Additions Charged to Costs and Expenses	(1,163)	(1,055)	(1,012)
Deduction	410	469	830
Balance at End of Year	(3,108)	(2,355)	(1,769)
Tax Valuation Allowance
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Year	(4,989)	(4,422)	(3,786)
Additions Charged to Costs and Expenses		(567)	(636)
Deduction	345	0	0
Balance at End of Year	$ (4,644)	$ (4,989)	$ (4,422)