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:
Balance at Beginning of Period	$ 3,527	$ 2,310	$ 2,135
Additions Charged to Costs and Expenses	2,452	4,632	2,515
Write-offs	4,037	3,935	3,024
Recoveries	674	520	684
Balance at End of Period	$ 2,616	$ 3,527	$ 2,310