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Investments - Summary of Investments (Detail) - USD ($)
$ in Thousands
Sep. 30, 2015
Dec. 31, 2014
Summary of Investment Holdings [Line Items]    
Amortized Cost $ 186,123 $ 63,795
Gross Unrealized Gains 91 3
Gross Unrealized Loss (46) (25)
Fair Value 186,168 63,773
Current [Member] | Certificates of Deposit [Member]    
Summary of Investment Holdings [Line Items]    
Amortized Cost   15,000
Fair Value   15,000
Current [Member] | Corporate Debt Securities [Member]    
Summary of Investment Holdings [Line Items]    
Amortized Cost 111,232 10,144
Gross Unrealized Gains 30 1
Gross Unrealized Loss (23) (10)
Fair Value 111,239 10,135
Current [Member] | Commercial Paper [Member]    
Summary of Investment Holdings [Line Items]    
Amortized Cost 40,465 29,980
Gross Unrealized Gains 33  
Fair Value 40,498 29,980
Non-current [Member] | Corporate Debt Securities (One to Two Years) [Member]    
Summary of Investment Holdings [Line Items]    
Amortized Cost 33,426 5,122
Gross Unrealized Gains 27 1
Gross Unrealized Loss (23) (10)
Fair Value 33,430 5,113
Non-current [Member] | US Government and Agency Securities [Member]    
Summary of Investment Holdings [Line Items]    
Amortized Cost 1,000 3,549
Gross Unrealized Gains 1 1
Gross Unrealized Loss   (5)
Fair Value $ 1,001 $ 3,545