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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 Year $ 12,252 $ 11,175 $ 10,424
Costs and Expenses 7,264 6,194 2,853
Deductions [1] (2,286) (5,117) (2,102)
Balance at End of Year $ 17,230 $ 12,252 $ 11,175
[1] Deductions primarily represent uncollectible balances of accounts receivable written off, net of recoveries.