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Schedule II-Valuation and Qualifying Accounts (Details) - Allowance for Doubtful Accounts - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Changes in valuation and qualifying accounts      
Balance at beginning of period $ 1,149 $ 1,552 $ 1,853
Additions:      
Bad debt expense (277) 1,213 1,450
Deductions:      
Charge-offs of uncollectible balances (397) (1,616) (1,751)
Balance at end of period $ 475 $ 1,149 $ 1,552