Quarterly report pursuant to Section 13 or 15(d)

Short Term Investments - Schedule of Available for Sale Short-term Investments by Major Security Types (Details)

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Short Term Investments - Schedule of Available for Sale Short-term Investments by Major Security Types (Details) - USD ($)
$ in Thousands
Mar. 31, 2019
Dec. 31, 2018
Schedule Of Available For Sale Securities [Line Items]    
Available for sale, Cost Total   $ 39,722
Available for sale, Unrealized gains   287
Available for sale, Unrealized (losses)   (9)
Available for sale, Fair Value Total   40,000
Available for sale, Maturity by period Less than 1 year   40,000
Commercial Paper    
Schedule Of Available For Sale Securities [Line Items]    
Available for sale, Cost Total $ 4,986 35,745
Available for sale, Unrealized gains 11 272
Available for sale, Unrealized (losses)   (9)
Available for sale, Fair Value Total 4,997 36,008
Available for sale, Maturity by period Less than 1 year $ 4,997 36,008
U.S. Treasury and Agency Bonds    
Schedule Of Available For Sale Securities [Line Items]    
Available for sale, Cost Total   3,977
Available for sale, Unrealized gains   15
Available for sale, Fair Value Total   3,992
Available for sale, Maturity by period Less than 1 year   $ 3,992