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Fair Value - Financial Assets and Liabilities Measured at Fair Value (Details) - USD ($)
$ in Thousands
Mar. 31, 2019
Dec. 31, 2018
Assets    
Marketable equity securities $ 1,259 $ 1,585
Level 1    
Assets    
Short-term investments 9,595 10,223
Marketable equity securities 1,259 1,585
Level 2    
Assets    
Short-term investments
Marketable equity securities
Level 3    
Assets    
Short-term investments
Marketable equity securities