Valuation and Qualifying Accounts (Details) - Allowance for Doubtful Accounts [Member] - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Period		$ 5,805	$ 5,369	$ 3,002
Additions Charged to Costs and Expenses		10,263	4,160	3,443
Deductions	[1]	(2,710)	(3,724)	(1,076)
Balance at End of Period		$ 13,358	$ 5,805	$ 5,369

[1] Uncollectible accounts written off, net of recoveries.