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, 2019
USD ($)
security
Dec. 31, 2018
USD ($)
security
Available-for-sale securities    
Proceeds from the maturity or redemption of available-for-sale debt securities $ 125,303,000 $ 138,038,000
Realized (gain) on available-for-sale debt securities $ 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  
Cash equivalents    
Available-for-sale securities    
Amortized cost $ 16,822,000  
Aggregate estimated fair value 16,822,000  
Marketable securities    
Available-for-sale securities    
Amortized cost 39,092,000 136,911,000
Gross unrealized gains 39,000 7,000
Gross unrealized losses (1,000) (163,000)
Aggregate estimated fair value 39,130,000 136,755,000
Corporate debt securities | Marketable securities    
Available-for-sale securities    
Amortized cost   102,818,000
Gross unrealized gains   5,000
Gross unrealized losses   (120,000)
Aggregate estimated fair value   102,703,000
Fair market value of investments in an unrealized loss position $ 2,013,000 $ 117,179,000
Number of investments in an unrealized loss position | security 1 37
Unrealized loss $ (1,000) $ (163,000)
Corporate Debt Securities Maturity Period Less Than Three Months | Marketable securities    
Available-for-sale securities    
Amortized cost 23,479,000  
Gross unrealized gains 7,000  
Gross unrealized losses (1,000)  
Aggregate estimated fair value 23,485,000  
Corporate Debt Securities Maturity Period Three Months To One Year | Marketable securities    
Available-for-sale securities    
Amortized cost 15,613,000  
Gross unrealized gains 32,000  
Aggregate estimated fair value $ 15,645,000  
Corporate Debt Securities Maturity Period Three Months To One Year | Minimum | Marketable securities    
Available-for-sale securities    
Available for sale securities debt maturity period   3 months
Corporate Debt Securities Maturity Period Three Months To One Year | Maximum | Marketable securities    
Available-for-sale securities    
Available for sale securities debt maturity period 3 months 1 year
Corporate Debt Securities Maturity Period One Year To Two Years | Marketable securities    
Available-for-sale securities    
Amortized cost   $ 23,153,000
Gross unrealized gains  
Gross unrealized losses   (43,000)
Aggregate estimated fair value   $ 23,110,000
Corporate Debt Securities Maturity Period One Year To Two Years | Minimum | Marketable securities    
Available-for-sale securities    
Available for sale securities debt maturity period 3 months 1 year
Corporate Debt Securities Maturity Period One Year To Two Years | Maximum | Marketable securities    
Available-for-sale securities    
Available for sale securities debt maturity period 1 year 2 years
Agency Bond Maturity Period Three Months To One Year | Marketable securities    
Available-for-sale securities    
Amortized cost   $ 3,963,000
Gross unrealized gains   2,000
Aggregate estimated fair value   $ 3,965,000
Agency Bond Maturity Period Three Months To One Year | Minimum | Marketable securities    
Available-for-sale securities    
Available for sale securities debt maturity period   3 months
Agency Bond Maturity Period Three Months To One Year | Maximum | Marketable securities    
Available-for-sale securities    
Available for sale securities debt maturity period   1 year
Treasury Bills Maturity Period Three Months To One Year | Marketable securities    
Available-for-sale securities    
Amortized cost   $ 1,980,000
Gross unrealized gains  
Aggregate estimated fair value   $ 1,980,000
Treasury Bills Maturity Period Three Months To One Year | Minimum | Marketable securities    
Available-for-sale securities    
Available for sale securities debt maturity period   3 months
Treasury Bills Maturity Period Three Months To One Year | Maximum | Marketable securities    
Available-for-sale securities    
Available for sale securities debt maturity period   1 year
Certificate of Deposit Maturity Period Three Months To One Year | Marketable securities    
Available-for-sale securities    
Amortized cost   $ 3,002,000
Gross unrealized gains  
Aggregate estimated fair value   $ 3,002,000
Certificate of Deposit Maturity Period Three Months To One Year | Minimum | Marketable securities    
Available-for-sale securities    
Available for sale securities debt maturity period   3 months
Certificate of Deposit Maturity Period Three Months To One Year | Maximum | Marketable securities    
Available-for-sale securities    
Available for sale securities debt maturity period   1 year
Commercial Paper Maturity Period Three Months to One Year | Marketable securities    
Available-for-sale securities    
Amortized cost   $ 1,995,000
Gross unrealized gains  
Aggregate estimated fair value   $ 1,995,000
Commercial Paper Maturity Period Three Months to One Year | Minimum | Marketable securities    
Available-for-sale securities    
Available for sale securities debt maturity period   3 months
Commercial Paper Maturity Period Three Months to One Year | Maximum | Marketable securities    
Available-for-sale securities    
Available for sale securities debt maturity period   1 year
Money market funds | Cash equivalents    
Available-for-sale securities    
Amortized cost $ 16,822,000  
Aggregate estimated fair value $ 16,822,000  
Money Market Funds Maturity Period Less Than Three Months | Maximum | Cash equivalents    
Available-for-sale securities    
Available for sale securities debt maturity period 3 months