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Supplementary Balance Sheet Information (Details 1) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Changes in allowance for doubtful accounts      
Balance at beginning of year $ 0 $ 245 $ 0
Increase (decrease) during the year 0 (245) 245
Balance at end of year $ 0 $ 0 $ 245