Schedule II - Valuation and Qualifying Accounts (Details) - Allowance for doubtful accounts - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Valuation and Qualifying Accounts
Balance at Beginning of Period	$ 4,574	$ 3,664	$ 3,088
Charged to Cost and Expenses	823	2,456	2,682
Charged to Other Accounts	0	0	0
Deductions, Actual Charge-Offs	(1,214)	(1,546)	(2,106)
Balance at End of Period	$ 4,183	$ 4,574	$ 3,664