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Marketable Securities - Schedule of Available-for-sale Marketable Securities and Cash and Cash Equivalents (Details)
$ in Thousands
9 Months Ended
Sep. 30, 2017
USD ($)
security
Dec. 31, 2016
USD ($)
security
Sep. 30, 2016
USD ($)
Dec. 31, 2015
USD ($)
Available-for-sale marketable securities and cash and cash equivalents        
Cash and cash equivalents, Amortized Cost Value $ 446,938 $ 258,567 $ 198,565 $ 159,678
Cash and cash equivalents, Gross Unrealized Gains 0 0    
Cash and cash equivalents, Gross Unrealized Losses 0 0    
Cash and cash equivalents, Fair Value 446,938 258,567    
Marketable securities, Amortized Cost Value 21,142 73,808    
Marketable securities, Gross Unrealized Gains 2 78    
Marketable securities, Gross Unrealized Losses 0 (6)    
Marketable securities, Fair Value $ 21,144 $ 73,880    
Debt securities held in an unrealized loss position for more than 12 months | security 0 0    
Debt securities held in an unrealized loss position for less than 12 months | security 3 13    
Fair value of debt securities in an unrealized loss position for less than 12 months $ 9,400 $ 35,700    
Marketable securities, contractual term to maturity 1 year      
Domestic corporate debt securities        
Available-for-sale marketable securities and cash and cash equivalents        
Marketable securities, Amortized Cost Value $ 11,655 19,317    
Marketable securities, Gross Unrealized Gains 0 0    
Marketable securities, Gross Unrealized Losses 0 (2)    
Marketable securities, Fair Value 11,655 19,315    
Domestic corporate commercial paper        
Available-for-sale marketable securities and cash and cash equivalents        
Marketable securities, Amortized Cost Value 9,487 31,852    
Marketable securities, Gross Unrealized Gains 2 78    
Marketable securities, Gross Unrealized Losses 0 0    
Marketable securities, Fair Value 9,489 31,930    
Asset-backed securities        
Available-for-sale marketable securities and cash and cash equivalents        
Marketable securities, Amortized Cost Value   22,639    
Marketable securities, Gross Unrealized Gains   0    
Marketable securities, Gross Unrealized Losses   (4)    
Marketable securities, Fair Value   22,635    
Cash        
Available-for-sale marketable securities and cash and cash equivalents        
Cash and cash equivalents, Amortized Cost Value 110,853 77,443    
Cash and cash equivalents, Fair Value 110,853 77,443    
Money market funds        
Available-for-sale marketable securities and cash and cash equivalents        
Cash and cash equivalents, Amortized Cost Value 315,110 173,631    
Cash and cash equivalents, Fair Value 315,110 173,631    
Domestic corporate debt securities        
Available-for-sale marketable securities and cash and cash equivalents        
Cash and cash equivalents, Amortized Cost Value 7,000 2,006    
Cash and cash equivalents, Gross Unrealized Gains 0 0    
Cash and cash equivalents, Gross Unrealized Losses 0 0    
Cash and cash equivalents, Fair Value 7,000 2,006    
Domestic corporate commercial paper        
Available-for-sale marketable securities and cash and cash equivalents        
Cash and cash equivalents, Amortized Cost Value 13,975 5,487    
Cash and cash equivalents, Gross Unrealized Gains 0 0    
Cash and cash equivalents, Gross Unrealized Losses 0 0    
Cash and cash equivalents, Fair Value $ 13,975 $ 5,487