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Schedule II - 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]      
Valuation Allowances and Reserves, Balance at Beginning of Period $ 16,116 $ 16,178 $ 15,347
Valuation Allowances and Reserves, Additions 1,912 2,207 2,640
Valuation Allowances and Reserves, Deductions 916 2,269 1,809
Valuation Allowances and Reserves, Balance at End of Period $ 17,112 $ 16,116 $ 16,178