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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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Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

+ References

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-Publisher FASB
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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.

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Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

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