Investments - Portfolio of marketable securities (Detail) (USD $) In Thousands, unless otherwise specified	6 Months Ended	12 Months Ended
Jun. 30, 2013	Dec. 31, 2012
Available-for-sale securities:
Cost	$ 230,094	$ 250,088
Gross Unrealized Gains	8,795	3,184
Gross Unrealized Losses	(2,243)	(2,257)
Estimated Fair Value	236,646	251,015
Cash equivalents cost	(76,128)	(51,887)
Cash equivalents estimated fair value	(76,128)	(51,887)
Marketable securities cost	153,966	198,201
Marketable securities unrealized gains	8,795	3,184
Marketable securities unrealized losses	(2,243)	(2,257)
Marketable securities estimated fair value	160,518	199,128
Short-term Deposits [Member]
Available-for-sale securities:
Cost	72,186	48,596
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	72,186	48,596
Mutual Funds [Member]
Available-for-sale securities:
Cost	10,758	10,368
Gross Unrealized Gains	3,154	1,452
Gross Unrealized Losses	(23)	0
Estimated Fair Value	13,889	11,820
US Treasury Securities [Member]
Available-for-sale securities:
Cost	49,477	99,299
Gross Unrealized Gains	83	178
Gross Unrealized Losses	(11)	0
Estimated Fair Value	49,549	99,477
Corporate Securities [Member]
Available-for-sale securities:
Cost	64,292	20,842
Gross Unrealized Gains	4,956	1,255
Gross Unrealized Losses	(2,120)	(1,980)
Estimated Fair Value	67,128	20,117
Corporate Obligations [Member]
Available-for-sale securities:
Cost	23,587	48,708
Gross Unrealized Gains	600	283
Gross Unrealized Losses	(89)	(277)
Estimated Fair Value	24,098	48,714
Commercial Paper [Member]
Available-for-sale securities:
Cost	9,794	22,275
Gross Unrealized Gains	2	16
Gross Unrealized Losses	0	0
Estimated Fair Value	$ 9,796	$ 22,291

Estimated fair value of marketable securities.

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- Definition

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.

+ References

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-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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- Definition

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

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- Definition

Amount of 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. Excludes cash and cash equivalents within disposal group and discontinued operation.

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-Section 02

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Total debt and equity financial instruments including: (1) securities held-to-maturity, (2) trading securities, and (3) securities available-for-sale.

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- Definition

The increase (decrease), resulting in a gain, in the difference between the fair value and the carrying value, or in the comparative fair values, of securities held at each balance sheet date, that was included in earnings for the period.

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The increase (decrease), resulting in a loss, in the difference between the fair value and the carrying value, or in the comparative fair values, of securities held at each balance sheet date, that was included in earnings for the period.

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