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
Sep. 30, 2018
Dec. 31, 2017
Schedule Of Available For Sale Securities [Line Items]    
Available for sale, Cost Total $ 68,194 $ 30,743
Available for sale, Unrealized gains 324 26
Available for sale, Unrealized (losses) (9) (38)
Available for sale, Fair Value Total 68,509 30,731
Available for sale, Maturity by period Less than 1 year 68,509 30,731
Commercial Paper    
Schedule Of Available For Sale Securities [Line Items]    
Available for sale, Cost Total 49,379 22,538
Available for sale, Unrealized gains 230 8
Available for sale, Unrealized (losses) (4) (27)
Available for sale, Fair Value Total 49,605 22,519
Available for sale, Maturity by period Less than 1 year 49,605 22,519
U.S. Treasury and Agency Bonds    
Schedule Of Available For Sale Securities [Line Items]    
Available for sale, Cost Total 18,815 8,205
Available for sale, Unrealized gains 94 18
Available for sale, Unrealized (losses) (5) (11)
Available for sale, Fair Value Total 18,904 8,212
Available for sale, Maturity by period Less than 1 year $ 18,904 $ 8,212