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Cash Equivalents and Investments - Schedule of Fair Value of Securities, not Including Cash (Details) - USD ($)
$ in Thousands
Sep. 30, 2017
Dec. 31, 2016
Cash And Cash Equivalents [Line Items]    
Amortized Cost $ 105,463 $ 111,334
Gross Unrealized Gains 2 11
Gross Unrealized Losses (30) (20)
Estimated Fair Value 105,435 111,325
Cash equivalents 18,418 45,937
Short-term investments 87,017 54,407
Long-term investments   10,981
Total cash equivalents and investments 105,435 111,325
Money Market Funds    
Cash And Cash Equivalents [Line Items]    
Amortized Cost 5,722 45,937
Estimated Fair Value 5,722 45,937
U.S. Government Securities    
Cash And Cash Equivalents [Line Items]    
Amortized Cost 35,942 16,479
Gross Unrealized Gains 1 11
Gross Unrealized Losses (26)  
Estimated Fair Value 35,917 16,490
Commercial Paper    
Cash And Cash Equivalents [Line Items]    
Amortized Cost 48,130 24,971
Estimated Fair Value 48,130 24,971
Corporate Notes    
Cash And Cash Equivalents [Line Items]    
Amortized Cost 15,669 23,947
Gross Unrealized Gains 1  
Gross Unrealized Losses (4) (20)
Estimated Fair Value $ 15,666 $ 23,927