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Fair Value Measurements - Narrative (Details) - USD ($)
Jun. 30, 2020
Dec. 31, 2019
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Amortized cost of available-for-sale investments with maturities less than one year $ 677,200,000 $ 450,200,000
Amortized cost of available-for-sale investments with maturities greater than one year 76,600,000 47,700,000
Net unrealized gains on investments, net of tax 1,200,000 100,000
Convertible Senior Notes    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Debt instrument, fair value 677,300,000  
Money market funds    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Unrealized gain 0 0
Unrealized loss $ 0 $ 0