Fair Value Measurements (Details) - USD ($)	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Fair Value Measurements
Other-than-temporary impairment	$ 0	$ 0
Transfer from Level 1 to Level 2, assets	0	0
Recurring
Fair Value Measurements
Assets fair value	366,693,000	347,923,000
Recurring \| Money market funds
Fair Value Measurements
Assets fair value	49,204,000	121,510,000
Recurring \| Overnight repurchase agreements
Fair Value Measurements
Assets fair value	11,250,000	24,000,000
Recurring \| U.S. government-sponsored enterprise securities
Fair Value Measurements
Assets fair value	18,181,000
Recurring \| Corporate debt securities
Fair Value Measurements
Assets fair value	148,874,000	47,906,000
Recurring \| Certificates of deposit
Fair Value Measurements
Assets fair value	7,794,000
Recurring \| Commercial paper obligations
Fair Value Measurements
Assets fair value	108,630,000	84,436,000
Recurring \| Asset-backed securities
Fair Value Measurements
Assets fair value	22,760,000	70,071,000
Recurring \| Quoted Prices in Active Markets (Level 1)
Fair Value Measurements
Assets fair value	49,204,000	121,510,000
Recurring \| Quoted Prices in Active Markets (Level 1) \| Money market funds
Fair Value Measurements
Assets fair value	49,204,000	121,510,000
Recurring \| Quoted Prices in Active Markets (Level 1) \| Overnight repurchase agreements
Fair Value Measurements
Assets fair value	0	0
Recurring \| Quoted Prices in Active Markets (Level 1) \| U.S. government-sponsored enterprise securities
Fair Value Measurements
Assets fair value	0
Recurring \| Quoted Prices in Active Markets (Level 1) \| Corporate debt securities
Fair Value Measurements
Assets fair value	0	0
Recurring \| Quoted Prices in Active Markets (Level 1) \| Certificates of deposit
Fair Value Measurements
Assets fair value	0
Recurring \| Quoted Prices in Active Markets (Level 1) \| Commercial paper obligations
Fair Value Measurements
Assets fair value	0	0
Recurring \| Quoted Prices in Active Markets (Level 1) \| Asset-backed securities
Fair Value Measurements
Assets fair value	0	0
Recurring \| Significant Other Observable Inputs (Level 2)
Fair Value Measurements
Assets fair value	317,489,000	226,413,000
Recurring \| Significant Other Observable Inputs (Level 2) \| Money market funds
Fair Value Measurements
Assets fair value	0	0
Recurring \| Significant Other Observable Inputs (Level 2) \| Overnight repurchase agreements
Fair Value Measurements
Assets fair value	11,250,000	24,000,000
Recurring \| Significant Other Observable Inputs (Level 2) \| U.S. government-sponsored enterprise securities
Fair Value Measurements
Assets fair value	18,181,000
Recurring \| Significant Other Observable Inputs (Level 2) \| Corporate debt securities
Fair Value Measurements
Assets fair value	148,874,000	47,906,000
Recurring \| Significant Other Observable Inputs (Level 2) \| Certificates of deposit
Fair Value Measurements
Assets fair value	7,794,000
Recurring \| Significant Other Observable Inputs (Level 2) \| Commercial paper obligations
Fair Value Measurements
Assets fair value	108,630,000	84,436,000
Recurring \| Significant Other Observable Inputs (Level 2) \| Asset-backed securities
Fair Value Measurements
Assets fair value	22,760,000	70,071,000
Recurring \| Significant Other Unobservable Inputs (Level 3)
Fair Value Measurements
Assets fair value	0	0
Recurring \| Significant Other Unobservable Inputs (Level 3) \| Money market funds
Fair Value Measurements
Assets fair value	0	0
Recurring \| Significant Other Unobservable Inputs (Level 3) \| Overnight repurchase agreements
Fair Value Measurements
Assets fair value	0	0
Recurring \| Significant Other Unobservable Inputs (Level 3) \| U.S. government-sponsored enterprise securities
Fair Value Measurements
Assets fair value	0
Recurring \| Significant Other Unobservable Inputs (Level 3) \| Corporate debt securities
Fair Value Measurements
Assets fair value	0	0
Recurring \| Significant Other Unobservable Inputs (Level 3) \| Certificates of deposit
Fair Value Measurements
Assets fair value	0
Recurring \| Significant Other Unobservable Inputs (Level 3) \| Commercial paper obligations
Fair Value Measurements
Assets fair value	0	0
Recurring \| Significant Other Unobservable Inputs (Level 3) \| Asset-backed securities
Fair Value Measurements
Assets fair value	$ 0	$ 0

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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- Definition

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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- Definition

The amount by which the fair value of an investment is less than the amortized cost basis or carrying amount of that investment at the balance sheet date and the decline in fair value is deemed to be other than temporary, before considering whether or not such amount is recognized in earnings or other comprehensive income.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3602-108585

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 35
-Paragraph 34
-URI http://asc.fasb.org/extlink&oid=82895926&loc=d3e25351-111560

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