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
Jun. 30, 2018
Dec. 31, 2017
Schedule Of Available For Sale Securities [Line Items]    
Available for sale, Cost Total $ 50,308 $ 30,743
Available for sale, Unrealized gains 65 26
Available for sale, Unrealized (losses) (3) (38)
Available for sale, Fair Value Total 50,370 30,731
Available for sale, Maturity by period Less than 1 year 50,370 30,731
Commercial Paper    
Schedule Of Available For Sale Securities [Line Items]    
Available for sale, Cost Total 47,831 22,538
Available for sale, Unrealized gains 58 8
Available for sale, Unrealized (losses) (3) (27)
Available for sale, Fair Value Total 47,886 22,519
Available for sale, Maturity by period Less than 1 year 47,886 22,519
U.S. Treasury and Agency Bonds    
Schedule Of Available For Sale Securities [Line Items]    
Available for sale, Cost Total 2,477 8,205
Available for sale, Unrealized gains 7 18
Available for sale, Unrealized (losses)   (11)
Available for sale, Fair Value Total 2,484 8,212
Available for sale, Maturity by period Less than 1 year $ 2,484 $ 8,212