Quarterly report pursuant to Section 13 or 15(d)

Available-for-sale debt securities (Details)

v3.10.0.1
Available-for-sale debt securities (Details)
 3 Months Ended 6 Months Ended
Jun. 30, 2018
USD ($)
security
Jun. 30, 2018
USD ($)
security
Dec. 31, 2017
USD ($)
security
Schedule of Available-for-sale Securities [Line Items]      
Realized loss on available-for-sale debt securities $ 1,310,000 $ 2,473,000  
Aggregate fair value of securities $ 81,714,000 $ 81,714,000 $ 125,828,000
Number of debt securities | security 33 33 54
Marketable securities      
Schedule of Available-for-sale Securities [Line Items]      
Amortized cost $ 86,850,000 $ 86,850,000  
Gross unrealized losses (134,000) (134,000)  
Aggregate estimated fair value 86,716,000 86,716,000  
Corporate Debt Securities Maturity Period Three Months To One Year | Marketable securities      
Schedule of Available-for-sale Securities [Line Items]      
Amortized cost 77,820,000 77,820,000  
Gross unrealized losses (132,000) (132,000)  
Aggregate estimated fair value 77,688,000 77,688,000  
Government Bonds Maturity Period Three Months to One Year | Marketable securities      
Schedule of Available-for-sale Securities [Line Items]      
Amortized cost 6,030,000 6,030,000  
Gross unrealized losses (2,000) (2,000)  
Aggregate estimated fair value 6,028,000 6,028,000  
Commercial Paper Maturity Period Three Months to One Year | Marketable securities      
Schedule of Available-for-sale Securities [Line Items]      
Amortized cost 3,000,000 3,000,000  
Aggregate estimated fair value 3,000,000 3,000,000  
Debt Securities Available For Sale Continuous Unrealized Loss Position More than one year      
Schedule of Available-for-sale Securities [Line Items]      
Aggregate estimated fair value 1,400,000 1,400,000  
Aggregate fair value of securities $ 2,000 $ 2,000  
Number of debt securities | security 1 1  
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 | Government Bonds 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 | Commercial Paper 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 | Government Bonds 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 | Commercial Paper 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