Schedule II - Valuation and qualifying accounts (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Allowance for Doubtful Accounts
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Year	$ (3,108)	$ (2,355)	$ (1,769)
Additions Charged to Costs and Expenses	(3,115)	(1,163)	(1,055)
Deduction	910	410	469
Balance at End of Year	(5,313)	(3,108)	(2,355)
Tax Valuation Allowance
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Year	(4,644)	(4,989)	(4,422)
Additions Charged to Costs and Expenses	(5,569)		(567)
Deduction		345
Balance at End of Year	$ (10,213)	$ (4,644)	$ (4,989)