Annual report pursuant to Section 13 and 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
Dec. 31, 2018
Dec. 31, 2017
Schedule Of Available For Sale Securities [Line Items]    
Available for sale, Cost Total $ 39,722 $ 30,743
Available for sale, Unrealized gains 287 26
Available for sale, Unrealized (losses) (9) (38)
Available for sale, Fair Value Total 40,000 30,731
Available for sale, Maturity by period Less than 1 year 40,000 30,731
Commercial Paper    
Schedule Of Available For Sale Securities [Line Items]    
Available for sale, Cost Total 35,745 22,538
Available for sale, Unrealized gains 272 8
Available for sale, Unrealized (losses) (9) (27)
Available for sale, Fair Value Total 36,008 22,519
Available for sale, Maturity by period Less than 1 year 36,008 22,519
U.S. Treasury and Agency Bonds    
Schedule Of Available For Sale Securities [Line Items]    
Available for sale, Cost Total 3,977 8,205
Available for sale, Unrealized gains 15 18
Available for sale, Unrealized (losses)   (11)
Available for sale, Fair Value Total 3,992 8,212
Available for sale, Maturity by period Less than 1 year $ 3,992 $ 8,212