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Fair Value Measurement - Assets Measured at Fair Value on a Recurring Basis (Details) - USD ($)
Sep. 30, 2020
Dec. 31, 2019
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Estimated Fair Value $ 280,965,000 $ 301,083,000
Fair value of assets measured on a recurring basis 346,509,000 421,032,000
Money market funds    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Money market funds 65,544,000 119,949,000
Asset-backed securities    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Estimated Fair Value 30,741,000 30,539,000
Corporate debt securities    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Estimated Fair Value 108,839,000 161,472,000
U.S. Treasury securities    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Estimated Fair Value 90,368,000 75,227,000
Commercial paper    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Estimated Fair Value 51,017,000 33,845,000
Level 3    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Investments, Fair Value Disclosure 0 0
Level 1    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair value of assets measured on a recurring basis 155,912,000 195,176,000
Level 1 | Money market funds    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Money market funds 65,544,000 119,949,000
Level 1 | Asset-backed securities    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Estimated Fair Value 0 0
Level 1 | Corporate debt securities    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Estimated Fair Value 0 0
Level 1 | U.S. Treasury securities    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Estimated Fair Value 90,368,000 75,227,000
Level 1 | Commercial paper    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Estimated Fair Value 0 0
Level 2    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair value of assets measured on a recurring basis 190,597,000 225,856,000
Level 2 | Money market funds    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Money market funds 0 0
Level 2 | Asset-backed securities    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Estimated Fair Value 30,741,000 30,539,000
Level 2 | Corporate debt securities    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Estimated Fair Value 108,839,000 161,472,000
Level 2 | U.S. Treasury securities    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Estimated Fair Value 0 0
Level 2 | Commercial paper    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Estimated Fair Value $ 51,017,000 $ 33,845,000