Cash Equivalents and Marketable Securities - Summary of Estimated Fair Value of Cash Equivalents and Marketable Securities (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost, Cash equivalents	$ 157,631	$ 246,323
Gross Unrealized Holding Gains, Cash equivalents	0	0
Gross Unrealized Holding Losses, Cash equivalents	0	0
Estimated Fair Value, Cash equivalents	157,631	246,323
Corporate debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost, Cash equivalents		1,702
Gross Unrealized Holding Gains, Cash equivalents		0
Gross Unrealized Holding Losses, Cash equivalents		0
Estimated Fair Value, Cash equivalents		1,702
Marketable securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost, Marketable securities	231,463	79,730
Gross Unrealized Holding Gains, Marketable securities	26
Gross Unrealized Holding Losses, Marketable securities	(53)	(58)
Estimated Fair Value, Marketable securities	231,436	79,672
Marketable securities [Member] \| Corporate debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost, Marketable securities	68,666	29,863
Gross Unrealized Holding Gains, Marketable securities	8
Gross Unrealized Holding Losses, Marketable securities	(31)	(37)
Estimated Fair Value, Marketable securities	68,643	29,826
Long-term marketable securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost, Marketable securities	52,023	38,442
Gross Unrealized Holding Gains, Marketable securities	14	1
Gross Unrealized Holding Losses, Marketable securities	(5)	(32)
Estimated Fair Value, Marketable securities	52,032	38,411
Long-term marketable securities [Member] \| Corporate debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost, Marketable securities	6,545	3,544
Gross Unrealized Holding Gains, Marketable securities	6	1
Gross Unrealized Holding Losses, Marketable securities		(7)
Estimated Fair Value, Marketable securities	6,551	3,538
Money market funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost, Cash equivalents	117,556	223,745
Gross Unrealized Holding Gains, Cash equivalents	0	0
Gross Unrealized Holding Losses, Cash equivalents	0	0
Estimated Fair Value, Cash equivalents	117,556	223,745
Commercial paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost, Cash equivalents	32,044	13,294
Gross Unrealized Holding Gains, Cash equivalents	0	0
Gross Unrealized Holding Losses, Cash equivalents	0	0
Estimated Fair Value, Cash equivalents	32,044	13,294
Commercial paper [Member] \| Marketable securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost, Marketable securities	12,496	1,999
Gross Unrealized Holding Gains, Marketable securities	0
Gross Unrealized Holding Losses, Marketable securities	0
Estimated Fair Value, Marketable securities	12,496	1,999
U.S. government and agency securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost, Cash equivalents	8,031	7,582
Gross Unrealized Holding Gains, Cash equivalents	0	0
Gross Unrealized Holding Losses, Cash equivalents	0	0
Estimated Fair Value, Cash equivalents	8,031	7,582
U.S. government and agency securities [Member] \| Marketable securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost, Marketable securities	150,301	47,868
Gross Unrealized Holding Gains, Marketable securities	18
Gross Unrealized Holding Losses, Marketable securities	(22)	(21)
Estimated Fair Value, Marketable securities	150,297	47,847
U.S. government and agency securities [Member] \| Long-term marketable securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost, Marketable securities	45,478	34,898
Gross Unrealized Holding Gains, Marketable securities	8
Gross Unrealized Holding Losses, Marketable securities	(5)	(25)
Estimated Fair Value, Marketable securities	$ 45,481	$ 34,873

Cash and cash equivalents amortized cost.

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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on cash equivalents.

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on cash equivalents.

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

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Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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