Fair Value of Financial Instruments - Schedule of Fair Value Hierarchy of Financial Assets Carried at Fair Value (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial assets, Fair Value	$ 2,391,562	$ 314,474
Level I [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial assets, Fair Value	2,391,562	314,474
Money market funds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial assets, Fair Value	2,391,562	297,810
Money market funds [Member] | Level I [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial assets, Fair Value	$ 2,391,562	297,810
U.S. treasury bills [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial assets, Fair Value		16,664
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		$ 16,664