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Revenue - Contract Balances (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Jun. 30, 2018
Sep. 30, 2018
Dec. 31, 2017
Revenue      
Contract liabilities $ (927) $ (648) $ (924)
Amounts included in contract liabilities at the beginning of the period $ 425 $ 776