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Net Income (Loss) Per Share - Narrative (Details) - 2014 Warrants - shares | 3 Months Ended |
--- | --- | ---
Mar. 30, 2018 | Dec. 31, 2016
Class of Warrant or Right [Line Items] |  |
Term of warrants | 2 years |
Warrant outstanding (in shares) |  | 1,000,000

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- Definition Class of Warrant or Right, Period + References No definition available. + Details
Name: | exel_ClassofWarrantorRightPeriod
Namespace Prefix: | exel_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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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. + Details
Name: | us-gaap_ClassOfWarrantOrRightLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Number of warrants or rights outstanding. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-08.(i)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690 + Details
Name: | us-gaap_ClassOfWarrantOrRightOutstanding
Namespace Prefix: | us-gaap_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | instant

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- Details
Name: | us-gaap_ClassOfWarrantOrRightAxis=exel_WarrantsIssuedOnJanuaryTwentySecondTwoThousandAndFourteenMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |