Quarterly report pursuant to Section 13 or 15(d)

Available-for-sale debt securities (Details)

v3.8.0.1
Available-for-sale debt securities (Details)
 3 Months Ended
Mar. 31, 2018
USD ($)
security
Dec. 31, 2017
USD ($)
security
Schedule of Available-for-sale Securities [Line Items]    
Realized loss on available-for-sale debt securities $ 1,163,000  
Aggregate fair value of securities $ 110,979,000 $ 125,828,000
Number of debt securities | security 46 54
Cash equivalents    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost $ 2,520,000  
Gross Unrealized Losses (42,000)  
Foreign currency translation adjustment 42,000  
Aggregate Estimated Fair Value 2,520,000  
Marketable securities    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 108,718,000  
Gross Unrealized Losses (6,887,000)  
Foreign currency translation adjustment 6,628,000  
Aggregate Estimated Fair Value 108,459,000  
Corporate Debt Securities Maturity Period Less Than Three Months | Cash equivalents    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 2,520,000  
Gross Unrealized Losses (42,000)  
Foreign currency translation adjustment 42,000  
Aggregate Estimated Fair Value 2,520,000  
Corporate Debt Securities Maturity Period Three Months To One Year | Marketable securities    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 99,724,000  
Gross Unrealized Losses (6,729,000)  
Foreign currency translation adjustment 6,470,000  
Aggregate Estimated Fair Value 99,465,000  
Government Bonds Maturity Period Three Months to One Year | Marketable securities    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 6,007,000  
Gross Unrealized Losses (108,000)  
Foreign currency translation adjustment 108,000  
Aggregate Estimated Fair Value 6,007,000  
Commercial Paper Maturity Period Three Months to One Year | Marketable securities    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 2,987,000  
Gross Unrealized Losses (50,000)  
Foreign currency translation adjustment 50,000  
Aggregate Estimated Fair Value $ 2,987,000  
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 | 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 Less Than Three Months | Cash equivalents    
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