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Fair Value Measurements - Fair value hierarchy of assets and liabilities (Details) - Fair Value, Measurements, Recurring - USD ($)
$ in Thousands
Mar. 31, 2020
Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Money market funds $ 30,811 $ 30,673
Equity securities 4,865 23,878
NAV Practical Expedient (No Fair Value Level)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Money market funds 0 0
Equity securities 696 15,068
Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Money market funds 30,811 30,673
Equity securities 4,169 8,810
Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Money market funds 0 0
Equity securities 0 0
Level 3    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Money market funds 0 0
Equity securities $ 0 $ 0