Available-for-Sale Securities (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended
Jun. 30, 2012	Dec. 31, 2011
Available-for-sale securities
Amortized Cost	$ 377,596	$ 235,734
Gross Unrealized Gains	14	36
Gross Unrealized Losses	(123)	(19)
Estimated Fair Value	377,487	235,751
Maturity period for marketable securities
Average contractual maturity period	11 months
Maximum
Maturity period for marketable securities
Maximum contractual maturity period	24 months
Cash and cash equivalents
Available-for-sale securities
Estimated Fair Value	159,053	38,721
Short-term investments
Available-for-sale securities
Estimated Fair Value	134,393	196,137
Long-term marketable securities
Available-for-sale securities
Estimated Fair Value	83,208
Restricted cash
Available-for-sale securities
Estimated Fair Value	833	893
U.S. government securities
Available-for-sale securities
Amortized Cost	30,270	66,150
Gross Unrealized Gains	1	24
Gross Unrealized Losses	(16)
Estimated Fair Value	30,255	66,174
U.S. government agencies
Available-for-sale securities
Amortized Cost	112,266	93,183
Gross Unrealized Gains	4	9
Gross Unrealized Losses	(74)	(17)
Estimated Fair Value	112,196	93,175
U.S. corporate notes
Available-for-sale securities
Amortized Cost	60,176	2,707
Gross Unrealized Gains	9
Gross Unrealized Losses	(33)	(2)
Estimated Fair Value	60,152	2,705
U.S. commercial paper
Available-for-sale securities
Amortized Cost	44,993	34,973
Gross Unrealized Gains		3
Estimated Fair Value	44,993	34,976
Money market funds
Available-for-sale securities
Amortized Cost	129,891	38,721
Estimated Fair Value	$ 129,891	$ 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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