Fair Value Measurements (Details) - USD ($)	Dec. 31, 2016	Dec. 31, 2015
Fair Value Measurements
Overnight repurchase agreements	$ 24,000,000
Transfer from Level 1 to Level 2, assets	0	$ 0
Recurring \| Fair value
Fair Value Measurements
Assets fair value	347,923,000	342,660,000
Recurring \| Money market funds and overnight repurchase agreements \| Fair value
Fair Value Measurements
Assets fair value	145,510,000	54,077,000
Recurring \| U.S. government-sponsored enterprise securities \| Fair value
Fair Value Measurements
Assets fair value		24,290,000
Recurring \| Corporate debt securities \| Fair value
Fair Value Measurements
Assets fair value	47,906,000	73,651,000
Recurring \| Commercial paper obligations \| Fair value
Fair Value Measurements
Assets fair value	84,436,000	125,805,000
Recurring \| Asset-backed securities \| Fair value
Fair Value Measurements
Assets fair value	70,071,000	64,837,000
Recurring \| Quoted Prices in Active Markets (Level 1)
Fair Value Measurements
Assets fair value	121,510,000	30,077,000
Recurring \| Quoted Prices in Active Markets (Level 1) \| Money market funds and overnight repurchase agreements
Fair Value Measurements
Assets fair value	121,510,000	30,077,000
Recurring \| Significant Other Observable Inputs (Level 2)
Fair Value Measurements
Assets fair value	226,413,000	312,583,000
Recurring \| Significant Other Observable Inputs (Level 2) \| Money market funds and overnight repurchase agreements
Fair Value Measurements
Assets fair value	24,000,000	24,000,000
Recurring \| Significant Other Observable Inputs (Level 2) \| U.S. government-sponsored enterprise securities
Fair Value Measurements
Assets fair value		24,290,000
Recurring \| Significant Other Observable Inputs (Level 2) \| Corporate debt securities
Fair Value Measurements
Assets fair value	47,906,000	73,651,000
Recurring \| Significant Other Observable Inputs (Level 2) \| Commercial paper obligations
Fair Value Measurements
Assets fair value	84,436,000	125,805,000
Recurring \| Significant Other Observable Inputs (Level 2) \| Asset-backed securities
Fair Value Measurements
Assets fair value	$ 70,071,000	$ 64,837,000

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://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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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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Amount of transfers of assets measured on a recurring basis out of Level 1 of the fair value hierarchy into Level 2.

+ 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 (bb)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

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Investment in short-term money-market instruments (such as commercial paper, banker's acceptances, repurchase agreements, government securities, certificates of deposit, and so forth) which are highly liquid (that is, readily convertible to known amounts of cash) and so near their maturity that they present an insignificant risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify as cash equivalents by definition. Original maturity means an original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three-years ago does not become a cash equivalent when its remaining maturity is three months.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Cash Equivalents
-URI http://asc.fasb.org/extlink&oid=6507016

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