Fair Value Measurements (Details) (Assets and liabilities measured on a recurring basis, USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Level 1
Major categories of assets and liabilities measured on a recurring basis, at fair value
Total	$ 180,161	$ 278,698
Level 1 \| United States treasuries
Major categories of assets and liabilities measured on a recurring basis, at fair value
Cash equivalents and short-term investments	180,161	278,698
Level 2
Major categories of assets and liabilities measured on a recurring basis, at fair value
Total	149,327	123,054
Level 2 \| Commercial paper
Major categories of assets and liabilities measured on a recurring basis, at fair value
Cash equivalents and short-term investments	11,943
Level 2 \| Corporate bonds
Major categories of assets and liabilities measured on a recurring basis, at fair value
Cash equivalents and short-term investments	74,721
Level 2 \| Government agency securities
Major categories of assets and liabilities measured on a recurring basis, at fair value
Cash equivalents and short-term investments	62,663	123,054
Fair Value
Major categories of assets and liabilities measured on a recurring basis, at fair value
Total	329,488	401,752
Fair Value \| United States treasuries
Major categories of assets and liabilities measured on a recurring basis, at fair value
Cash equivalents and short-term investments	180,161	278,698
Fair Value \| Commercial paper
Major categories of assets and liabilities measured on a recurring basis, at fair value
Cash equivalents and short-term investments	11,943
Fair Value \| Corporate bonds
Major categories of assets and liabilities measured on a recurring basis, at fair value
Cash equivalents and short-term investments	74,721
Fair Value \| Government agency securities
Major categories of assets and liabilities measured on a recurring basis, at fair value
Cash equivalents and short-term investments	$ 62,663	$ 123,054

- Definition

Cash includes currency on hand as well as demand deposits with banks or financial institutions. It also includes other kinds of accounts that have the general characteristics of demand deposits in that the customer may deposit additional funds at any time and effectively may withdraw funds at any time without prior notice or penalty. Cash equivalents, excluding items classified as marketable securities, include short-term, highly liquid Investments that are both readily convertible to known amounts of cash, and so near their maturity that they present minimal risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify under that definition. Original maturity means original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three years ago does not become a cash equivalent when its remaining maturity is three months. Short-term investments, exclusive of cash equivalents, generally consist of marketable securities intended to be sold within one year (or the normal operating cycle if longer) and may include trading securities, available-for-sale securities, or held-to-maturity securities (if maturing within one year), as applicable.

+ References+ Details