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Fair Value Measurements - Fair value hierarchy of assets and liabilities (Details) - Fair Value, Measurements, Recurring - USD ($)
$ in Thousands
Sep. 30, 2019
Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Money market funds $ 64,601 $ 57,790
Equity securities 22,438 18,109
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 13,991 12,252
Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Money market funds 64,601 57,790
Equity securities 8,447 5,857
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