Financial Instruments - Summary of Marketable Securities (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Cost	$ 332,843	$ 395,464
Available-for-sale securities, Gross Unrealized Gains	86	257
Available-for-sale securities, Gross Unrealized Losses	(193)	(138)
Estimated Fair Value	332,736	395,583
Corporate Bonds And Notes
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Cost	133,045	171,485
Available-for-sale securities, Gross Unrealized Gains	46	177
Available-for-sale securities, Gross Unrealized Losses	(143)	(98)
Estimated Fair Value	132,948	171,564
Municipal Bonds And Notes
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Cost	118,262	110,185
Available-for-sale securities, Gross Unrealized Gains	32	55
Available-for-sale securities, Gross Unrealized Losses	(38)	(32)
Estimated Fair Value	118,256	110,208
Commercial Paper
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Cost	44,800	46,135
Available-for-sale securities, Gross Unrealized Gains	7	12
Available-for-sale securities, Gross Unrealized Losses	(1)	(4)
Estimated Fair Value	44,806	46,143
Treasury And Agency Notes And Bills
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Cost	17,402	29,050
Available-for-sale securities, Gross Unrealized Gains	0	12
Available-for-sale securities, Gross Unrealized Losses	(11)	(4)
Estimated Fair Value	17,391	29,058
Money Market Funds
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Cost	15,834	34,106
Available-for-sale securities, Gross Unrealized Gains	0	0
Available-for-sale securities, Gross Unrealized Losses	0	0
Estimated Fair Value	15,834	34,106
Certificates of Deposit
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Cost	3,500	4,503
Available-for-sale securities, Gross Unrealized Gains	1	1
Available-for-sale securities, Gross Unrealized Losses	0	0
Estimated Fair Value	3,501	4,504
Cash and Cash Equivalents
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value	29,002	56,782
Short-term Investments
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value	$ 303,734	$ 338,801

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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This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents Available-for-sale Securities which consist of all investments in certain debt and equity securities neither classified as trading or held-to-maturity securities. A debt security represents a creditor relationship with an enterprise. Debt securities include, among other items, US Treasury securities, US government securities, municipal securities, corporate bonds, convertible debt, commercial paper, and all securitized debt instruments. An equity security represents an ownership interest in an enterprise or the right to acquire or dispose of an ownership interest in an enterprise at fixed or determinable prices. Equity securities include, among other things, common stock, certain preferred stock, warrant rights, call options, and put options, but do not include convertible debt. An entity may opt to provide the reader with additional narrative text to better understand the nature of investments in debt and equity securities which are categorized as Available-for-sale.

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- Definition

This item represents the gross unrealized gains for securities, at a point in time, which are categorized neither as held-to-maturity nor trading securities.

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