Annual report pursuant to Section 13 and 15(d)

Summary of Significant Accounting Policies - Available-for-sale debt securities (Details)

v3.20.4
Summary of Significant Accounting Policies - Available-for-sale debt securities (Details)
 12 Months Ended
Dec. 31, 2020
USD ($)
security
Dec. 31, 2019
USD ($)
security
Dec. 31, 2018
USD ($)
Marketable securities      
Proceeds from the maturity or redemption of available-for-sale debt securities $ 105,022,000 $ 125,303,000 $ 138,038,000
Realized gain on available-for-sale debt securities $ 89,000 13,000  
Realized loss on available-for-sale debt securities     $ 2,473,000
Number of available-for-sale securities in an unrealized loss position for more than one year 0    
Marketable securities      
Marketable securities      
Amortized cost $ 311,223,000 39,092,000  
Gross unrealized gains 268,000 39,000  
Gross unrealized losses (156,000) (1,000)  
Aggregate estimated fair value 311,335,000 39,130,000  
Corporate debt securities | Marketable securities      
Marketable securities      
Fair market value of investments in an unrealized loss position $ 157,985,000 $ 2,013,000  
Number of investments in an unrealized loss position | security 30 1  
Unrealized loss $ (158,000) $ (1,000)  
Corporate Debt Securities Maturity Period Less Than Three Months | Marketable securities      
Marketable securities      
Amortized cost 67,545,000 23,479,000  
Gross unrealized gains 56,000 7,000  
Gross unrealized losses (20,000) (1,000)  
Aggregate estimated fair value $ 67,581,000 $ 23,485,000  
Corporate Debt Securities Maturity Period Less Than Three Months | Maximum | Marketable securities      
Marketable securities      
Available for sale securities debt maturity period 3 months 3 months  
Corporate Debt Securities Maturity Period Three Months To One Year | Marketable securities      
Marketable securities      
Amortized cost $ 101,447,000 $ 15,613,000  
Gross unrealized gains 92,000 32,000  
Gross unrealized losses (24,000)    
Aggregate estimated fair value $ 101,515,000 $ 15,645,000  
Corporate Debt Securities Maturity Period Three Months To One Year | Minimum | Marketable securities      
Marketable securities      
Available for sale securities debt maturity period 3 months 3 months  
Corporate Debt Securities Maturity Period Three Months To One Year | Maximum | Marketable securities      
Marketable securities      
Available for sale securities debt maturity period 1 year 1 year  
Corporate Debt Securities Maturity Period One Year To Two Years | Marketable securities      
Marketable securities      
Amortized cost $ 136,238,000    
Gross unrealized gains 112,000    
Gross unrealized losses (112,000)    
Aggregate estimated fair value $ 136,238,000    
Corporate Debt Securities Maturity Period One Year To Two Years | Minimum | Marketable securities      
Marketable securities      
Available for sale securities debt maturity period 1 year    
Corporate Debt Securities Maturity Period One Year To Two Years | Maximum | Marketable securities      
Marketable securities      
Available for sale securities debt maturity period 2 years    
Agency Bond Maturity Period One Year To Two Years | Marketable securities      
Marketable securities      
Amortized cost $ 5,993,000    
Gross unrealized gains 8,000    
Aggregate estimated fair value $ 6,001,000    
Agency Bond Maturity Period One Year To Two Years | Minimum | Marketable securities      
Marketable securities      
Available for sale securities debt maturity period 1 year    
Agency Bond Maturity Period One Year To Two Years | Maximum | Marketable securities      
Marketable securities      
Available for sale securities debt maturity period 2 years