Aggregate Fair Value, Amortized Cost Basis and Net Unrealized Gains and Losses of Available-for-Sale Investments (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011	Jun. 30, 2011
Schedule of Available-for-sale Securities [Line Items]
Aggregate Fair Value	$ 0	$ 148,488	$ 147,136
Amortized Cost	0	148,467	147,142
Net Unrealized Gains (Losses)	0	21	(6)
Government Obligations
Schedule of Available-for-sale Securities [Line Items]
Aggregate Fair Value	0	100,517	100,679
Amortized Cost	0	100,480	100,664
Net Unrealized Gains (Losses)	0	37	15
Government Agency Obligations
Schedule of Available-for-sale Securities [Line Items]
Aggregate Fair Value	0	31,351	26,235
Amortized Cost	0	31,354	26,237
Net Unrealized Gains (Losses)	0	(3)	(2)
Corporate Obligations
Schedule of Available-for-sale Securities [Line Items]
Aggregate Fair Value	0	16,620	20,222
Amortized Cost	0	16,633	20,241
Net Unrealized Gains (Losses)	$ 0	$ (13)	$ (19)

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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