Annual report pursuant to Section 13 and 15(d)

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

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Summary of Significant Accounting Policies - Available-for-sale debt securities (Details)
 12 Months Ended
Dec. 31, 2021
USD ($)
security
Dec. 31, 2020
USD ($)
security
Dec. 31, 2019
USD ($)
Marketable securities      
Proceeds from the maturity or redemption of available-for-sale debt securities $ 224,343,000 $ 105,022,000 $ 125,303,000
Realized gain on available-for-sale debt securities 6,000 89,000 $ 13,000
Fair market value of investments in an unrealized loss position $ 176,483,000 $ 157,985,000  
Number of available-for-sale securities in an unrealized loss position for less than 12 months | security 36 30  
Unrealized losses, 12 mo. or longer $ (35,000)    
Unrealized losses, less than 12 mo. (390,000) $ (158,000)  
Marketable securities      
Marketable securities      
Amortized cost 219,989,000 311,223,000  
Gross unrealized gains 33,000 268,000  
Gross unrealized losses (390,000) (156,000)  
Aggregate estimated fair value 219,632,000 311,335,000  
Corporate Debt Securities Maturity Period Less Than Three Months | Marketable securities      
Marketable securities      
Amortized cost 45,304,000 67,545,000  
Gross unrealized gains 22,000 56,000  
Gross unrealized losses (21,000) (20,000)  
Aggregate estimated fair value $ 45,305,000 $ 67,581,000  
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 $ 81,590,000 $ 101,447,000  
Gross unrealized gains 11,000 92,000  
Gross unrealized losses (75,000) (24,000)  
Aggregate estimated fair value $ 81,526,000 $ 101,515,000  
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 $ 88,095,000 $ 136,238,000  
Gross unrealized gains   112,000  
Gross unrealized losses (287,000) (112,000)  
Aggregate estimated fair value $ 87,808,000 $ 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 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 2 years  
Agency Bond Maturity Period One Year To Two Years | Marketable securities      
Marketable securities      
Amortized cost $ 5,000,000 $ 5,993,000  
Gross unrealized gains   8,000  
Gross unrealized losses (7,000)    
Aggregate estimated fair value $ 4,993,000 $ 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 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 2 years  
Agency Bond Continuous Loss Position For Less Than 12 Months | Marketable securities      
Marketable securities      
Fair market value of investments in an unrealized loss position $ 4,993,000    
Number of available-for-sale securities in an unrealized loss position for less than 12 months | security 1    
Unrealized losses, less than 12 mo. $ (7,000)    
Corporate Debt Securities Continuous Loss Position For 12 Months Or Longer | Marketable securities      
Marketable securities      
Fair market value of investments in an unrealized loss position $ 8,232,000    
Number of available-for-sale securities in an unrealized loss position for more than one year | security 1    
Unrealized losses, 12 mo. or longer $ (35,000)    
Corporate Debt Securities Continuous Loss Position For Less Than 12 Months | Marketable securities      
Marketable securities      
Fair market value of investments in an unrealized loss position $ 163,258,000 $ 157,985,000  
Number of available-for-sale securities in an unrealized loss position for less than 12 months | security 34 30  
Unrealized losses, less than 12 mo. $ (348,000) $ (158,000)