Fair Value of Financial Instruments - Summary of Amortized Cost and Estimated Fair Values (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 422,858	$ 346,618
Unrealized Gains
Unrealized Gains (Losses)	29	(22)
Fair Value	422,887	346,596
U.S. Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	422,858	346,618
Unrealized Gains
Unrealized Gains (Losses)	29	(22)
Fair Value	$ 422,887	$ 346,596

Amount of debt securities categorized neither as held-to-maturity nor trading.

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