Fair Value of Financial Instruments and Non-Financial Assets - Measured on a Nonrecurring Basis (Details) (Fair Value, Measurements, Recurring [Member], USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Total Available-for-sale Debt Securities - Fair Value	$ 0	$ 0
Level 1 [Member] \| Money Market Funds [Member]
Assets, Fair Value Disclosure [Abstract]
Money market funds	0	0
Level 1 [Member] \| Commercial Paper [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale Securities	0	0
Level 1 [Member] \| U.S. Government or Government Agency Securities [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale Securities	0	0
Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Total Available-for-sale Debt Securities - Fair Value	510,941	344,824
Level 2 [Member] \| Money Market Funds [Member]
Assets, Fair Value Disclosure [Abstract]
Money market funds	12,144	126,617
Level 2 [Member] \| Commercial Paper [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale Securities	191,989	82,346
Level 2 [Member] \| U.S. Government or Government Agency Securities [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale Securities	306,808	135,861
Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Total Available-for-sale Debt Securities - Fair Value	0	0
Level 3 [Member] \| Money Market Funds [Member]
Assets, Fair Value Disclosure [Abstract]
Money market funds	0	0
Level 3 [Member] \| Commercial Paper [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale Securities	0	0
Level 3 [Member] \| U.S. Government or Government Agency Securities [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale Securities	0	0
Fair Value [Member]
Assets, Fair Value Disclosure [Abstract]
Total Available-for-sale Debt Securities - Fair Value	510,941	344,824
Fair Value [Member] \| Money Market Funds [Member]
Assets, Fair Value Disclosure [Abstract]
Money market funds	12,144	126,617
Fair Value [Member] \| Commercial Paper [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale Securities	191,989	82,346
Fair Value [Member] \| U.S. Government or Government Agency Securities [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale Securities	$ 306,808	$ 135,861

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 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 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 Company may deposit additional funds at any time and also 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.

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