Quarterly report pursuant to Section 13 or 15(d)

Available-for-sale debt securities (Details)

v3.19.2
Available-for-sale debt securities (Details)
 3 Months Ended 6 Months Ended
Jun. 30, 2019
USD ($)
security
Jun. 30, 2018
USD ($)
Jun. 30, 2019
USD ($)
security
Jun. 30, 2018
USD ($)
Dec. 31, 2018
USD ($)
security
Schedule of Available-for-sale Securities [Line Items]          
Realized (gain) on available-for-sale debt securities $ 13,000   $ 13,000    
Realized loss on available-for-sale debt securities   $ 1,310,000   $ 2,473,000  
Fair market value of investments in an unrealized loss position $ 9,600,000   $ 9,600,000   $ 117,200,000
Number of debt securities | security 3   3   37
Number of available-for-sale securities in an unrealized loss position for more than one year | security 0   0    
Cash equivalents          
Schedule of Available-for-sale Securities [Line Items]          
Amortized cost $ 10,241,000   $ 10,241,000    
Aggregate estimated fair value 10,241,000   10,241,000    
Marketable securities          
Schedule of Available-for-sale Securities [Line Items]          
Amortized cost 98,724,000   98,724,000    
Gross unrealized gains 110,000   110,000    
Gross unrealized losses (2,000)   (2,000)    
Aggregate estimated fair value 98,832,000   98,832,000    
Corporate Debt Securities Maturity Period Less Than Three Months | Marketable securities          
Schedule of Available-for-sale Securities [Line Items]          
Amortized cost 35,703,000   35,703,000    
Gross unrealized gains 18,000   18,000    
Aggregate estimated fair value 35,721,000   35,721,000    
Money Market Funds Maturity Period Less Than Three Months | Cash equivalents          
Schedule of Available-for-sale Securities [Line Items]          
Amortized cost 10,241,000   10,241,000    
Aggregate estimated fair value 10,241,000   10,241,000    
Corporate Debt Securities Maturity Period Three Months To One Year | Marketable securities          
Schedule of Available-for-sale Securities [Line Items]          
Amortized cost 56,027,000   56,027,000    
Gross unrealized gains 85,000   85,000    
Gross unrealized losses (2,000)   (2,000)    
Aggregate estimated fair value 56,110,000   56,110,000    
Agency Bond Maturity Period Three Months To One Year | Marketable securities          
Schedule of Available-for-sale Securities [Line Items]          
Amortized cost 3,992,000   3,992,000    
Gross unrealized gains 7,000   7,000    
Aggregate estimated fair value 3,999,000   3,999,000    
Certificate of Deposit Maturity Period Less Than Three Months | Marketable securities          
Schedule of Available-for-sale Securities [Line Items]          
Amortized cost 3,002,000   3,002,000    
Aggregate estimated fair value $ 3,002,000   $ 3,002,000    
Minimum | Money Market Funds Maturity Period Less Than Three Months | Cash equivalents          
Schedule of Available-for-sale Securities [Line Items]          
Available For Sale Securities Debt Maturity Period     3 years    
Minimum | Corporate Debt Securities Maturity Period Three Months To One Year | Marketable securities          
Schedule of Available-for-sale Securities [Line Items]          
Available For Sale Securities Debt Maturity Period     3 months    
Minimum | Agency Bond Maturity Period Three Months To One Year | Marketable securities          
Schedule of Available-for-sale Securities [Line Items]          
Available For Sale Securities Debt Maturity Period     3 months    
Minimum | Certificate of Deposit Maturity Period Three Months To One Year | Marketable securities          
Schedule of Available-for-sale Securities [Line Items]          
Available For Sale Securities Debt Maturity Period     3 months    
Maximum | Corporate Debt Securities Maturity Period Three Months To One Year | Marketable securities          
Schedule of Available-for-sale Securities [Line Items]          
Available For Sale Securities Debt Maturity Period     1 year    
Maximum | Agency Bond Maturity Period Three Months To One Year | Marketable securities          
Schedule of Available-for-sale Securities [Line Items]          
Available For Sale Securities Debt Maturity Period     1 year