Fair Value of Financial Instruments - Schedule of Fair Value Hierarchy of Financial Assets Carried at Fair Value (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial assets, Fair Value	$ 1,701,806	$ 460,313
Money market funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial assets, Fair Value	1,685,123	460,313
U.S. treasury bills [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial assets, Fair Value	16,683
Level I [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial assets, Fair Value	1,701,806	460,313
Level I [Member] | Money market funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial assets, Fair Value	1,685,123	460,313
Level I [Member] | U.S. treasury bills [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial assets, Fair Value	$ 16,683