Schedule II - Valuation and Qualifying Accounts - (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounts receivable allowance
Valuation allowance and reserves
Balance at Beginning of Period	$ 8,753	$ 8,370	$ 5,748
Additions Charged to Expense	838	1,332	2,180
Charged to Other Accounts	9,658	12,960	13,505
Charges Utilized (Write-off)	(10,590)	(13,909)	(13,063)
Balance at End of Period	8,659	8,753	8,370
Noncurrent other assets allowance
Valuation allowance and reserves
Balance at Beginning of Period	5,737	4,554	3,081
Charged to Other Accounts	6,918	3,209	2,826
Charges Utilized (Write-off)	(5,468)	(2,026)	(1,353)
Balance at End of Period	$ 7,187	$ 5,737	$ 4,554