Cash and Investments in Marketable Securities - Portfolio of Cash and Investments in Marketable Securities (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale investments	$ 225,459	$ 197,959
Cash, including restricted cash	26,136	37,614
Total cash and investments in marketable securities	262,824	262,026
Corporate Notes and Bonds [Member] | Fair Value Hierarchy Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale investments	182,544	138,515
Corporate Commercial Paper [Member] | Fair Value Hierarchy Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale investments	42,915	59,444
Money Market Funds [Member] | Fair Value Hierarchy Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds	$ 11,229	$ 26,453