Summary of Significant Accounting Policies - Fair Value (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Fair Value of Financial Instruments
Money Market Funds	$ 26,246	$ 16,528
Securities	504,223
Money Market Funds
Fair Value of Financial Instruments
Money Market Funds	18,270	5,175
Corporate Securities
Fair Value of Financial Instruments
Securities	104,967	123,270
Government Securities
Fair Value of Financial Instruments
Securities	399,256	223,530
Fair Value, Measurements, Recurring
Fair Value of Financial Instruments
Total Fair Value	522,493	351,975
Fair Value, Measurements, Recurring \| Money Market Funds
Fair Value of Financial Instruments
Money Market Funds	18,270	5,175
Fair Value, Measurements, Recurring \| Corporate Securities
Fair Value of Financial Instruments
Securities	104,967	123,270
Fair Value, Measurements, Recurring \| Government Securities
Fair Value of Financial Instruments
Securities	399,256	223,530
Level 1 \| Fair Value, Measurements, Recurring
Fair Value of Financial Instruments
Total Fair Value	18,270	5,175
Level 1 \| Fair Value, Measurements, Recurring \| Money Market Funds
Fair Value of Financial Instruments
Money Market Funds	18,270	5,175
Level 2 \| Fair Value, Measurements, Recurring
Fair Value of Financial Instruments
Total Fair Value	504,223	346,800
Level 2 \| Fair Value, Measurements, Recurring \| Corporate Securities
Fair Value of Financial Instruments
Securities	104,967	123,270
Level 2 \| Fair Value, Measurements, Recurring \| Government Securities
Fair Value of Financial Instruments
Securities	$ 399,256	$ 223,530

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

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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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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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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