Fair Value Measurements (Details) - USD ($)	Sep. 30, 2018	Dec. 31, 2017
U.S. treasury securities
Assets:
Available-for-sale securities	$ 1,372,627,000	$ 561,327,000
U.S. agency securities
Assets:
Available-for-sale securities	17,649,000	17,663,000
Time deposits
Assets:
Time deposits		18,924,000
Recurring basis
Liabilities:
Liabilities measured and recorded at fair value	0	0
Recurring basis \| (Level 1)
Assets:
Total	1,661,739,000	642,644,000
Recurring basis \| (Level 1) \| Money market funds
Assets:
Cash equivalents	111,726,000	44,730,000
Recurring basis \| (Level 1) \| U.S. treasury securities
Assets:
Available-for-sale securities	1,372,627,000	561,327,000
Cash equivalents	159,737,000
Recurring basis \| (Level 1) \| U.S. agency securities
Assets:
Available-for-sale securities	17,649,000	17,663,000
Recurring basis \| (Level 1) \| Time deposits
Assets:
Time deposits		18,924,000
Recurring basis \| (Level 2)
Assets:
Total	0	0
Recurring basis \| (Level 2) \| Money market funds
Assets:
Cash equivalents	0	0
Recurring basis \| (Level 2) \| U.S. treasury securities
Assets:
Available-for-sale securities	0	0
Cash equivalents	0
Recurring basis \| (Level 2) \| U.S. agency securities
Assets:
Available-for-sale securities	0	0
Recurring basis \| (Level 2) \| Time deposits
Assets:
Time deposits		0
Recurring basis \| (Level 3)
Assets:
Total	0	0
Recurring basis \| (Level 3) \| Money market funds
Assets:
Cash equivalents	0	0
Recurring basis \| (Level 3) \| U.S. treasury securities
Assets:
Available-for-sale securities	0	0
Cash equivalents	0
Recurring basis \| (Level 3) \| U.S. agency securities
Assets:
Available-for-sale securities	$ 0	0
Recurring basis \| (Level 3) \| Time deposits
Assets:
Time deposits		$ 0

Time deposits, fair value disclosure.

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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://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

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- References

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- Definition

Amount of investments in debt securities classified as available-for-sale, expected to be converted to cash, sold or exchanged within one year or the normal operating cycle, if longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26626-111562

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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

No definition available.

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- Definition

Fair value of financial and nonfinancial obligations.

+ References

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