Fair Value Measurements (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair Value Measurements
Marketable securities	$ 503,503	$ 409,898
Total Assets Measured at Fair Value	685,523	556,780
Recurring \| Fair value
Fair Value Measurements
Marketable securities	503,503	409,898
Cash, Cash Equivalents and Restricted Cash	181,792	155,009
Total Assets Measured at Fair Value	685,295	564,907
Recurring \| Fair value \| Bonds and bond funds
Fair Value Measurements
Marketable securities	344,251	409,885
Recurring \| Fair value \| Commercial paper
Fair Value Measurements
Marketable securities	159,252	13
Recurring \| Level 1
Fair Value Measurements
Cash, Cash Equivalents and Restricted Cash	181,792	155,009
Total Assets Measured at Fair Value	181,792	155,009
Recurring \| Level 2
Fair Value Measurements
Marketable securities	503,503	409,898
Total Assets Measured at Fair Value	503,503	409,898
Recurring \| Level 2 \| Bonds and bond funds
Fair Value Measurements
Marketable securities	344,251	409,885
Recurring \| Level 2 \| Commercial paper
Fair Value Measurements
Marketable securities	$ 159,252	$ 13

This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item includes cash including restricted cash and cash equivalents.

No definition available.

+ Details

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Balance Type:	debit
Period Type:	instant

- 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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- Definition

Amount of investment in debt and equity securities categorized neither as trading securities nor held-to-maturity securities and intended be sold or mature one year or 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

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.

+ References

No definition available.

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