Available-for-Sale Securities (Details) (USD $) In Thousands, unless otherwise specified	9 Months Ended
Sep. 30, 2012	Dec. 31, 2011
Available-for-sale securities
Amortized Cost	$ 358,875	$ 235,734
Gross Unrealized Gains	124	36
Gross Unrealized Losses	(8)	(19)
Estimated Fair Value	358,991	235,751
Maturity period for marketable securities
Average contractual maturity period	9 months
Maximum
Maturity period for marketable securities
Maximum contractual maturity period	2 years
Cash and cash equivalents
Available-for-sale securities
Estimated Fair Value	107,987	38,721
Short-term investments
Available-for-sale securities
Estimated Fair Value	161,209	196,137
Long-term marketable securities
Available-for-sale securities
Estimated Fair Value	88,962
Restricted cash
Available-for-sale securities
Estimated Fair Value	833	893
U.S. government securities
Available-for-sale securities
Amortized Cost	30,233	66,150
Gross Unrealized Gains	9	24
Estimated Fair Value	30,242	66,174
U.S. government agencies
Available-for-sale securities
Amortized Cost	107,073	93,183
Gross Unrealized Gains	69	9
Gross Unrealized Losses	(1)	(17)
Estimated Fair Value	107,141	93,175
U.S. corporate notes
Available-for-sale securities
Amortized Cost	98,599	2,707
Gross Unrealized Gains	45
Gross Unrealized Losses	(7)	(2)
Estimated Fair Value	98,637	2,705
U.S. commercial paper
Available-for-sale securities
Amortized Cost	37,336	34,973
Gross Unrealized Gains	1	3
Estimated Fair Value	37,337	34,976
Money market funds
Available-for-sale securities
Amortized Cost	85,634	38,721
Estimated Fair Value	$ 85,634	$ 38,721

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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