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Revenue - Contract Balances (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Mar. 31, 2019
Jun. 30, 2019
Dec. 31, 2018
Revenue      
Contract liabilities $ (483) $ (395) $ (476)
Amounts included in contract liabilities at the beginning of the period $ 226 $ 322