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Fair Value Measurements - Narrative (Details) - USD ($)
Dec. 31, 2020
Dec. 31, 2019
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Restricted cash $ 6,660,000 $ 6,660,000
Amortized cost of available-for-sale investments with maturities less than one year 866,500,000 450,200,000
Amortized cost of available-for-sale investments with maturities greater than one year 56,500,000 47,700,000
Net unrealized gains (losses) on investments, net of tax 200,000 100,000
Convertible Debt    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Debt instrument, fair value 1,225,600,000  
Money market funds    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Unrealized gain 0 0
Unrealized loss $ 0 $ 0