Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Fair Value of Financial Instruments
Cash equivalents	$ 117,751	$ 400,304
Investments, available-for-sale	$ 498,900	$ 273,052
Investments, available-for-sale, type	us-gaap:USTreasurySecuritiesMember	us-gaap:USTreasurySecuritiesMember
Recurring
Fair Value of Financial Instruments
Cash equivalents	$ 117,751	$ 400,304
Investments, available-for-sale	498,900	273,052
Total	616,651	673,356
Recurring \| Active Markets (Level 1)
Fair Value of Financial Instruments
Cash equivalents	117,751	400,304
Investments, available-for-sale	498,900	273,052
Total	$ 616,651	$ 673,356

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

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

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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
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109979856&loc=d3e26610-111562

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-Publisher FASB
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-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
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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.

+ References

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- Definition

Indicates type 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/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

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