Annual report pursuant to Section 13 and 15(d)

Summary of Significant Accounting Policies - Available-for-sale debt securities (Details)

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Summary of Significant Accounting Policies - Available-for-sale debt securities (Details)
12 Months Ended
Dec. 31, 2018
USD ($)
security
Dec. 31, 2017
USD ($)
security
Available-for-sale securities    
Proceeds from the maturity or redemption of available-for-sale debt securities $ 138,038,000 $ 29,090,000
Realized loss on available-for-sale debt securities $ 2,473,000 646,000
Number of available-for-sale securities in an unrealized loss position for more than one year | security 0  
Marketable securities    
Available-for-sale securities    
Amortized cost $ 136,911,000 124,406,000
Gross unrealized gains 7,000
Gross unrealized losses (163,000) (3,723,000)
Foreign currency translation adjustment   3,535,000
Aggregate estimated fair value 136,755,000 124,218,000
Corporate debt securities    
Available-for-sale securities    
Fair market value of investments in an unrealized loss position $ 117,179,000 $ 125,828,000
Number of investments in an unrealized loss position | security 37 54
Unrealized loss $ (163,000) $ (3,745,000)
Corporate debt securities | Cash equivalents    
Available-for-sale securities    
Amortized cost   1,610,000
Gross unrealized gains  
Gross unrealized losses   (22,000)
Foreign currency translation adjustment   22,000
Aggregate estimated fair value   $ 1,610,000
Corporate debt securities | Minimum | Marketable securities    
Available-for-sale securities    
Available for sale securities debt maturity period   3 months
Corporate debt securities | Maximum | Cash equivalents    
Available-for-sale securities    
Available for sale securities debt maturity period   3 months
Corporate debt securities | Maximum | Marketable securities    
Available-for-sale securities    
Available for sale securities debt maturity period   1 year
Corporate Debt Securities Maturity Period Less Than Three Months | Cash equivalents    
Available-for-sale securities    
Amortized cost   $ 1,610,000
Gross unrealized gains  
Gross unrealized losses   (22,000)
Foreign currency translation adjustment   22,000
Aggregate estimated fair value   1,610,000
Corporate Debt Securities Maturity Period Three Months To One Year | Marketable securities    
Available-for-sale securities    
Amortized cost 102,818,000 124,406,000
Gross unrealized gains 5,000
Gross unrealized losses (120,000) (3,723,000)
Foreign currency translation adjustment   3,535,000
Aggregate estimated fair value $ 102,703,000 124,218,000
Corporate Debt Securities Maturity Period Three Months To One Year | Minimum | Marketable securities    
Available-for-sale securities    
Available for sale securities debt maturity period 3 months  
Corporate Debt Securities Maturity Period Three Months To One Year | Maximum | Marketable securities    
Available-for-sale securities    
Available for sale securities debt maturity period 1 year  
Corporate Debt Securities Maturity Period One Year To Two Years | Marketable securities    
Available-for-sale securities    
Amortized cost $ 23,153,000  
Gross unrealized losses (43,000)  
Aggregate estimated fair value $ 23,110,000  
Corporate Debt Securities Maturity Period One Year To Two Years | Minimum | Marketable securities    
Available-for-sale securities    
Available for sale securities debt maturity period 1 year  
Corporate Debt Securities Maturity Period One Year To Two Years | Maximum | Marketable securities    
Available-for-sale securities    
Available for sale securities debt maturity period 2 years  
Agency Bond Maturity Period Three Months To One Year | Marketable securities    
Available-for-sale securities    
Amortized cost $ 3,963,000  
Gross unrealized gains 2,000  
Aggregate estimated fair value $ 3,965,000  
Agency Bond Maturity Period Three Months To One Year | Minimum | Marketable securities    
Available-for-sale securities    
Available for sale securities debt maturity period 3 months  
Agency Bond Maturity Period Three Months To One Year | Maximum | Marketable securities    
Available-for-sale securities    
Available for sale securities debt maturity period 1 year  
Treasury Bills Maturity Period Three Months To One Year | Marketable securities    
Available-for-sale securities    
Amortized cost $ 1,980,000  
Aggregate estimated fair value $ 1,980,000  
Treasury Bills Maturity Period Three Months To One Year | Minimum | Marketable securities    
Available-for-sale securities    
Available for sale securities debt maturity period 3 months  
Treasury Bills Maturity Period Three Months To One Year | Maximum | Marketable securities    
Available-for-sale securities    
Available for sale securities debt maturity period 1 year  
Certificate of Deposit Maturity Period Three Months To One Year | Marketable securities    
Available-for-sale securities    
Amortized cost $ 3,002,000  
Aggregate estimated fair value $ 3,002,000  
Certificate of Deposit Maturity Period Three Months To One Year | Minimum | Marketable securities    
Available-for-sale securities    
Available for sale securities debt maturity period 3 months  
Certificate of Deposit Maturity Period Three Months To One Year | Maximum | Marketable securities    
Available-for-sale securities    
Available for sale securities debt maturity period 1 month  
Commercial Paper Maturity Period Three Months to One Year | Marketable securities    
Available-for-sale securities    
Amortized cost $ 1,995,000  
Gross unrealized gains  
Aggregate estimated fair value $ 1,995,000  
Commercial Paper Maturity Period Three Months to One Year | Minimum | Marketable securities    
Available-for-sale securities    
Available for sale securities debt maturity period 3 months 3 months
Commercial Paper Maturity Period Three Months to One Year | Maximum | Marketable securities    
Available-for-sale securities    
Available for sale securities debt maturity period 1 year 1 year