Fair Value Measurements - Recurring Basis (Details) $ in Thousands, item in Millions	9 Months Ended
Sep. 30, 2018 USD ($) item	Dec. 31, 2017 USD ($)
Financial Assets
Marketable securities	$ 340,918	$ 329,139
Transfers between levels \| item	0.0
Government agency securities
Financial Assets
Marketable securities	$ 29,907	58,950
Corporate equity securities
Financial Assets
Amortized Cost	3,667
Money market funds
Financial Assets
Marketable securities	106,974	134,635
U.S. Treasuries
Financial Assets
Marketable securities	200,370	135,554
Recurring
Financial Assets
Total financial assets	342,537	330,758
Recurring \| U.S. Treasuries
Financial Assets
Marketable securities	195,885	123,559
Recurring \| Government agency securities
Financial Assets
Marketable securities	29,907	58,950
Recurring \| Corporate equity securities
Financial Assets
Amortized Cost	3,667
Recurring \| Money market funds
Financial Assets
Cash and cash equivalents	106,974	134,635
Restricted cash	1,619	1,619
Recurring \| U.S. Treasuries
Financial Assets
Cash and cash equivalents	4,485	11,995
Recurring \| Level 1
Financial Assets
Total financial assets	338,870	330,758
Recurring \| Level 1 \| U.S. Treasuries
Financial Assets
Marketable securities	195,885	123,559
Recurring \| Level 1 \| Government agency securities
Financial Assets
Marketable securities	29,907	58,950
Recurring \| Level 1 \| Money market funds
Financial Assets
Cash and cash equivalents	106,974	134,635
Restricted cash	1,619	1,619
Recurring \| Level 1 \| U.S. Treasuries
Financial Assets
Cash and cash equivalents	4,485	$ 11,995
Recurring \| Level 2
Financial Assets
Total financial assets	3,667
Recurring \| Level 2 \| Corporate equity securities
Financial Assets
Amortized Cost	$ 3,667

The number of transfers between levels of the fair value hierarchy during the period.

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- Definition

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

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
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- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
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- Definition

Fair value portion 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.

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- Definition

Cost of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI). Excludes equity method investment and investment in equity security without readily determinable fair value.

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Amount of cash and cash equivalents restricted as to withdrawal or usage. Cash includes, but is not limited to, currency on hand, demand deposits with banks or financial institutions, and other accounts with general characteristics of demand deposits. Cash equivalents include, but are not limited to, 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.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
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