Fair Value of Financial Instruments - Schedule of Fair Value Hierarchy of Financial Assets Carried at Fair Value (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial assets, Fair Value	$ 634,023	$ 1,292,019
Level I [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial assets, Fair Value	634,023	1,292,019
Money market funds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial assets, Fair Value	617,350	1,275,346
Money market funds [Member] | Level I [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial assets, Fair Value	617,350	1,275,346
U.S. treasury bills [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial assets, Fair Value	16,673	16,673
U.S. treasury bills [Member] | Level I [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial assets, Fair Value	$ 16,673	$ 16,673