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Summary of Significant Accounting Policies - Fair Value (Details) - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014
--- | --- | --- | --- | ---
Fair Value of Financial Instruments |  |  |  |
Money Market Funds | $ 16,528 | $ 14,528 | $ 12,590 | $ 54,649
Fair Value, Measurements, Recurring |  |  |  |
Fair Value of Financial Instruments |  |  |  |
Total Fair Value | 351,975 | 401,085 |  |
Fair Value, Measurements, Recurring | Money Market Funds |  |  |  |
Fair Value of Financial Instruments |  |  |  |
Money Market Funds | 5,175 | 12,137 |  |
Fair Value, Measurements, Recurring | Corporate Securities |  |  |  |
Fair Value of Financial Instruments |  |  |  |
Securities | 123,270 | 181,483 |  |
Fair Value, Measurements, Recurring | Government Securities |  |  |  |
Fair Value of Financial Instruments |  |  |  |
Securities | 223,530 | 207,465 |  |
Level 1 | Fair Value, Measurements, Recurring |  |  |  |
Fair Value of Financial Instruments |  |  |  |
Total Fair Value | 5,175 | 12,137 |  |
Level 1 | Fair Value, Measurements, Recurring | Money Market Funds |  |  |  |
Fair Value of Financial Instruments |  |  |  |
Money Market Funds | 5,175 | 12,137 |  |
Level 2 | Fair Value, Measurements, Recurring |  |  |  |
Fair Value of Financial Instruments |  |  |  |
Total Fair Value | 346,800 | 388,948 |  |
Level 2 | Fair Value, Measurements, Recurring | Corporate Securities |  |  |  |
Fair Value of Financial Instruments |  |  |  |
Securities | 123,270 | 181,483 |  |
Level 2 | Fair Value, Measurements, Recurring | Government Securities |  |  |  |
Fair Value of Financial Instruments |  |  |  |
Securities | $ 223,530 | $ 207,465 |  |

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- Definition Amount of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Excludes cash and cash equivalents within disposal group and discontinued operation. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section 45 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=82887183&loc=d3e6676-107765 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3044-108585 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.1) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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