Quarterly report pursuant to Section 13 or 15(d)

Cash, Cash Equivalents and Investments - Summary of Available-for-sale Securities' Adjusted Cost, Gross Unrealized Gains, Gross Unrealized Losses and Fair Value by Significant Investment Category (Details)

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Cash, Cash Equivalents and Investments - Summary of Available-for-sale Securities' Adjusted Cost, Gross Unrealized Gains, Gross Unrealized Losses and Fair Value by Significant Investment Category (Details)
$ in Thousands
Sep. 30, 2018
USD ($)
Debt Securities, Available-for-sale [Line Items]  
Adjusted Cost $ 252,793
Unrealized Losses (13)
Fair Value 252,780
Money market funds  
Debt Securities, Available-for-sale [Line Items]  
Adjusted Cost 51,273
Unrealized Losses 0
Fair Value 51,273
Commercial paper  
Debt Securities, Available-for-sale [Line Items]  
Adjusted Cost 170,302
Unrealized Losses 0
Fair Value 170,302
Certificates of deposit  
Debt Securities, Available-for-sale [Line Items]  
Adjusted Cost 7,504
Unrealized Losses 0
Fair Value 7,504
Corporate debt securities  
Debt Securities, Available-for-sale [Line Items]  
Adjusted Cost 19,217
Unrealized Losses (13)
Fair Value 19,204
U.S. government and government agency debt securities  
Debt Securities, Available-for-sale [Line Items]  
Adjusted Cost 4,497
Unrealized Losses 0
Fair Value $ 4,497