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Investments (Details) - USD ($)
$ in Thousands
Dec. 31, 2016
Dec. 31, 2015
Summary of Investments    
Available-for-sale securities, Amortized Cost $ 305,148 $ 238,568
Available-for-sale securities, Gross Unrealized Gains 35 5
Available-for-sale securities, Gross Unrealized Losses (402) (947)
Available-for-sale securities, Estimated Fair Value 304,781 237,626
Corporate debt securities    
Summary of Investments    
Available-for-sale securities, due in one year or less, Amortized Cost 106,430 27,964
Available-for-sale securities, due in one to three years, Amortized Cost 139,742 173,652
Available-for-sale securities, due in one year or less, Gross Unrealized Gains 3 0
Available-for-sale securities, due in one to three years, Gross Unrealized Gains 32 3
Available-for-sale securities, due in one year or less, Gross Unrealized Losses (69) (38)
Available-for-sale securities, due in one to three years, Gross Unrealized Losses (281) (904)
Available-for-sale securities, due in one year or less, Estimated Fair Value 106,364 27,926
Available-for-sale securities, due in one to three years, Estimated Fair Value 139,493 172,751
U.S. treasury and government agency securities    
Summary of Investments    
Available-for-sale securities, due in one year or less, Amortized Cost 1,021  
Available-for-sale securities, due in one to three years, Amortized Cost 11,395  
Available-for-sale securities, due in one year or less, Gross Unrealized Gains 0  
Available-for-sale securities, due in one to three years, Gross Unrealized Gains 0  
Available-for-sale securities, due in one year or less, Gross Unrealized Losses 0  
Available-for-sale securities, due in one to three years, Gross Unrealized Losses (52)  
Available-for-sale securities, due in one year or less, Estimated Fair Value 1,021  
Available-for-sale securities, due in one to three years, Estimated Fair Value 11,343  
Commercial paper    
Summary of Investments    
Available-for-sale securities, due in one year or less, Amortized Cost 40,560 34,452
Available-for-sale securities, due in one year or less, Gross Unrealized Gains 0 2
Available-for-sale securities, due in one year or less, Gross Unrealized Losses 0 (5)
Available-for-sale securities, due in one year or less, Estimated Fair Value 40,560 34,449
Certificates of deposit    
Summary of Investments    
Available-for-sale securities, due in one year or less, Amortized Cost 6,000  
Available-for-sale securities, due in one year or less, Gross Unrealized Gains 0  
Available-for-sale securities, due in one year or less, Gross Unrealized Losses 0  
Available-for-sale securities, due in one year or less, Estimated Fair Value $ 6,000  
Municipal securities    
Summary of Investments    
Available-for-sale securities, due in one year or less, Amortized Cost   2,500
Available-for-sale securities, due in one year or less, Gross Unrealized Gains   0
Available-for-sale securities, due in one year or less, Gross Unrealized Losses   0
Available-for-sale securities, due in one year or less, Estimated Fair Value   $ 2,500