Quarterly report pursuant to Section 13 or 15(d)

Available-for-sale debt securities (Details)

v3.19.1
Available-for-sale debt securities (Details)
 3 Months Ended
Mar. 31, 2019
USD ($)
security
Mar. 31, 2018
USD ($)
Dec. 31, 2018
USD ($)
security
Schedule of Available-for-sale Securities [Line Items]      
Realized loss on available-for-sale debt securities $ 0 $ 1,163,000  
Fair market value of investments in an unrealized loss position $ 49,567,000   $ 117,179,000
Number of debt securities | security 15   37
Number of available-for-sale securities in an unrealized loss position for more than one year | security 0    
Cash equivalents      
Schedule of Available-for-sale Securities [Line Items]      
Amortized cost $ 7,990,000    
Aggregate estimated fair value 7,990,000    
Marketable securities      
Schedule of Available-for-sale Securities [Line Items]      
Amortized cost 118,187,000    
Gross unrealized gains 69,000    
Gross unrealized losses (15,000)    
Aggregate estimated fair value 118,241,000    
Corporate Debt Securities Maturity Period Less Than Three Months | Cash equivalents      
Schedule of Available-for-sale Securities [Line Items]      
Amortized cost 7,990,000    
Aggregate estimated fair value 7,990,000    
Corporate Debt Securities Maturity Period Three Months To One Year | Marketable securities      
Schedule of Available-for-sale Securities [Line Items]      
Amortized cost 109,203,000    
Gross unrealized gains 64,000    
Gross unrealized losses (15,000)    
Aggregate estimated fair value 109,252,000    
Agency Bond Maturity Period Three Months To One Year      
Schedule of Available-for-sale Securities [Line Items]      
Amortized cost 3,990,000    
Gross unrealized gains 5,000    
Aggregate estimated fair value 3,995,000    
Treasury Bills Maturity Period Three Months To One Year      
Schedule of Available-for-sale Securities [Line Items]      
Amortized cost 1,992,000    
Aggregate estimated fair value 1,992,000    
Certificate of Deposit Maturity Period Three Months To One Year      
Schedule of Available-for-sale Securities [Line Items]      
Amortized cost 3,002,000    
Aggregate estimated fair value $ 3,002,000    
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 | Treasury Bills 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    
Maximum | Treasury Bills 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 | 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 1 year