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Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified | Jun. 30, 2013 | Dec. 31, 2012
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Fair Value Measurements |  |
Amount of transfers of financial assets between Level 1 and Level 2 measures | $ 0 |
Assets: |  |
Short term investments: | 99,832 | 200,577
Available-for-sale equity securities |  |
Assets: |  |
Short term investments: | 78,662 | 174,610
Recurring |  |
Assets: |  |
Total short-term investments | 99,832 |
Recurring | Available-for-sale equity securities |  |
Assets: |  |
Short term investments: | 78,662 |
Recurring | Corporate debt securities |  |
Assets: |  |
Short term investments: | 19,685 |
Recurring | Mortgage-backed debt securities |  |
Assets: |  |
Short term investments: | 216 |
Recurring | Mortgage-CMO debt securities |  |
Assets: |  |
Short term investments: | 209 |
Recurring | Asset-backed debt securities |  |
Assets: |  |
Short term investments: | 1,060 |
Recurring | Level 1 |  |
Assets: |  |
Total short-term investments | 78,860 |
Recurring | Level 1 | Available-for-sale equity securities |  |
Assets: |  |
Short term investments: | 77,800 |
Recurring | Level 1 | Asset-backed debt securities |  |
Assets: |  |
Short term investments: | 1,060 |
Recurring | Level 2 |  |
Assets: |  |
Total short-term investments | 20,972 |
Recurring | Level 2 | Available-for-sale equity securities |  |
Assets: |  |
Short term investments: | 862 |
Recurring | Level 2 | Corporate debt securities |  |
Assets: |  |
Short term investments: | 19,685 |
Recurring | Level 2 | Mortgage-backed debt securities |  |
Assets: |  |
Short term investments: | 216 |
Recurring | Level 2 | Mortgage-CMO debt securities |  |
Assets: |  |
Short term investments: | $ 209 |

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- Definition Represents the amount of transfers of assets measured on a recurring basis between Level 1 and Level 2 of the fair value hierarchy. + References No definition available. + Details
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- Definition 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. + References No definition available. + Details
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- Definition 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. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 25 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=7534914&loc=d3e22054-111558 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 115 -Paragraph 12 -Subparagraph b -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Available-for-Sale Securities -URI http://asc.fasb.org/extlink&oid=6505594 + Details
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