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 9 Months Ended
Sep. 30, 2018
USD ($)
security
Sep. 30, 2018
USD ($)
security
Dec. 31, 2017
USD ($)
security
Schedule of Available-for-sale Securities [Line Items]      
Realized loss on available-for-sale debt securities $ 0 $ 2,473,000  
Aggregate fair value of securities $ 79,485,000 $ 79,485,000 $ 125,828,000
Number of debt securities | security 28 28 54
Marketable securities      
Schedule of Available-for-sale Securities [Line Items]      
Amortized cost $ 84,701,000 $ 84,701,000  
Gross unrealized losses (49,000) (49,000)  
Aggregate estimated fair value 84,652,000 84,652,000  
Corporate Debt Securities Maturity Period Three Months To One Year | Marketable securities      
Schedule of Available-for-sale Securities [Line Items]      
Amortized cost 82,717,000 82,717,000  
Gross unrealized losses (49,000) (49,000)  
Aggregate estimated fair value 82,668,000 82,668,000  
Commercial Paper Maturity Period Three Months to One Year | Marketable securities      
Schedule of Available-for-sale Securities [Line Items]      
Amortized cost 1,984,000 1,984,000  
Aggregate estimated fair value $ 1,984,000 $ 1,984,000  
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 | 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 | 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