Schedule II -- Valuation and Qualifying Accounts (Details) - Allowance for Doubtful Accounts - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Period	$ 44,730	$ 59,689	$ 66,405
Write-Off of Receivables	(30,515)	(41,567)	(49,160)
Charged to Costs and Expenses	23,697	26,608	42,444
Balance at End of Period	$ 37,912	$ 44,730	$ 59,689