    <SEC-DOCUMENT>0000875045-20-000032-index.html : 20201021
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    <ACCEPTANCE-DATETIME>20201021072159
    ACCESSION NUMBER:		0000875045-20-000032
    CONFORMED SUBMISSION TYPE:	8-K
    PUBLIC DOCUMENT COUNT:		16
    CONFORMED PERIOD OF REPORT:	20201021
    ITEM INFORMATION:		Results of Operations and Financial Condition
    ITEM INFORMATION:		Other Events
    ITEM INFORMATION:		Financial Statements and Exhibits
    FILED AS OF DATE:		20201021
    DATE AS OF CHANGE:		20201021

    FILER:

    	COMPANY DATA:
    		COMPANY CONFORMED NAME:			BIOGEN INC.
    		CENTRAL INDEX KEY:			0000875045
    		STANDARD INDUSTRIAL CLASSIFICATION:	BIOLOGICAL PRODUCTS (NO DIAGNOSTIC SUBSTANCES) [2836]
    		IRS NUMBER:				330112644
    		STATE OF INCORPORATION:			DE
    		FISCAL YEAR END:			1231

    	FILING VALUES:
    		FORM TYPE:		8-K
    		SEC ACT:		1934 Act
    		SEC FILE NUMBER:	000-19311
    		FILM NUMBER:		201249613

    	BUSINESS ADDRESS:
    		STREET 1:		225 BINNEY STREET
    		CITY:			CAMBRIDGE
    		STATE:			MA
    		ZIP:			02142
    		BUSINESS PHONE:		7814642000

    	MAIL ADDRESS:
    		STREET 1:		225 BINNEY STREET
    		CITY:			CAMBRIDGE
    		STATE:			MA
    		ZIP:			02142

    	FORMER COMPANY:
    		FORMER CONFORMED NAME:	BIOGEN IDEC INC.
    		DATE OF NAME CHANGE:	20070427

    	FORMER COMPANY:
    		FORMER CONFORMED NAME:	BIOGEN IDEC INC
    		DATE OF NAME CHANGE:	20031112

    	FORMER COMPANY:
    		FORMER CONFORMED NAME:	IDEC PHARMACEUTICALS CORP / DE
    		DATE OF NAME CHANGE:	19970530
    </SEC-HEADER>
    <DOCUMENT>
    <TYPE>8-K
    <SEQUENCE>1
    <FILENAME>biib-20201021.htm
    <DESCRIPTION>8-K
    <TEXT>
    Document 1 - file: biib-20201021.htm

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>EX-99.1
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    <FILENAME>exhibit991-q32020press.htm
    <DESCRIPTION>EX-99.1
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    Document 2 - file: exhibit991-q32020press.htm

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    <FILENAME>biib-20201021.xsd
    <DESCRIPTION>XBRL TAXONOMY EXTENSION SCHEMA DOCUMENT
    <TEXT>
    Document 3 - file: biib-20201021.xsd

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    <DESCRIPTION>XBRL TAXONOMY EXTENSION CALCULATION LINKBASE DOCUMENT
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    <FILENAME>biib-20201021_def.xml
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    Document 5 - file: biib-20201021_def.xml

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    <FILENAME>biib-20201021_lab.xml
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    <FILENAME>biogenlogostandarda2911.jpg
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    <FILENAME>R1.htm
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    <TYPE>EXCEL
    <SEQUENCE>11
    <FILENAME>Financial_Report.xlsx
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    <TEXT>
    Document 11 - file: Financial_Report.xlsx

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    <TEXT>
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    <TEXT>
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    <TYPE>XML
    <SEQUENCE>14
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    <TEXT>
    Document 14 - file: FilingSummary.html

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    <TYPE>JSON
    <SEQUENCE>16
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    <TEXT>
    Document 16 - file: MetaLinks.json

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    <TYPE>ZIP
    <SEQUENCE>17
    <FILENAME>0000875045-20-000032-xbrl.zip
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