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Marketable Securities Narrative (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Marketable Securities [Abstract]        
Interest income, money market deposits $ 32 $ 3 $ 32 $ 15