    <SEC-DOCUMENT>0000875045-21-000049-index.html : 20211020
    <SEC-HEADER>0000875045-21-000049.hdr.sgml : 20211020
    <ACCEPTANCE-DATETIME>20211020070959
    ACCESSION NUMBER:		0000875045-21-000049
    CONFORMED SUBMISSION TYPE:	8-K
    PUBLIC DOCUMENT COUNT:		14
    CONFORMED PERIOD OF REPORT:	20211020
    ITEM INFORMATION:		Results of Operations and Financial Condition
    ITEM INFORMATION:		Financial Statements and Exhibits
    FILED AS OF DATE:		20211020
    DATE AS OF CHANGE:		20211020

    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:		211332725

    	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-20211020.htm
    <DESCRIPTION>8-K
    <TEXT>
    Document 1 - file: biib-20211020.htm

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>EX-99.1
    <SEQUENCE>2
    <FILENAME>exhibit991-q32021pressrele.htm
    <DESCRIPTION>EX-99.1
    <TEXT>
    Document 2 - file: exhibit991-q32021pressrele.htm

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>EX-101.SCH
    <SEQUENCE>3
    <FILENAME>biib-20211020.xsd
    <DESCRIPTION>XBRL TAXONOMY EXTENSION SCHEMA DOCUMENT
    <TEXT>
    Document 3 - file: biib-20211020.xsd

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>EX-101.LAB
    <SEQUENCE>4
    <FILENAME>biib-20211020_lab.xml
    <DESCRIPTION>XBRL TAXONOMY EXTENSION LABEL LINKBASE DOCUMENT
    <TEXT>
    Document 4 - file: biib-20211020_lab.xml

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>EX-101.PRE
    <SEQUENCE>5
    <FILENAME>biib-20211020_pre.xml
    <DESCRIPTION>XBRL TAXONOMY EXTENSION PRESENTATION LINKBASE DOCUMENT
    <TEXT>
    Document 5 - file: biib-20211020_pre.xml

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>GRAPHIC
    <SEQUENCE>6
    <FILENAME>biogenlogostandarda29a.jpg
    <TEXT>
    Document 6 - file: biogenlogostandarda29a.jpg

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>XML
    <SEQUENCE>7
    <FILENAME>biib-20211020_htm.xml
    <DESCRIPTION>IDEA: XBRL DOCUMENT
    <TEXT>
    Document 7 - file: biib-20211020_htm.html

    Document 7 - RAW XML: biib-20211020_htm.xml

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    <DOCUMENT>
    <TYPE>XML
    <SEQUENCE>8
    <FILENAME>R1.htm
    <DESCRIPTION>IDEA: XBRL DOCUMENT
    <TEXT>
    Document 8 - file: R1.htm

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>EXCEL
    <SEQUENCE>9
    <FILENAME>Financial_Report.xlsx
    <DESCRIPTION>IDEA: XBRL DOCUMENT
    <TEXT>
    Document 9 - file: Financial_Report.xlsx

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>XML
    <SEQUENCE>10
    <FILENAME>Show.js
    <DESCRIPTION>IDEA: XBRL DOCUMENT
    <TEXT>
    Document 10 - file: Show.js

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>XML
    <SEQUENCE>11
    <FILENAME>report.css
    <DESCRIPTION>IDEA: XBRL DOCUMENT
    <TEXT>
    Document 11 - file: report.css

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>XML
    <SEQUENCE>12
    <FILENAME>FilingSummary.xml
    <DESCRIPTION>IDEA: XBRL DOCUMENT
    <TEXT>
    Document 12 - file: FilingSummary.html

    Document 12 - RAW XML: FilingSummary.xml

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>JSON
    <SEQUENCE>14
    <FILENAME>MetaLinks.json
    <DESCRIPTION>IDEA: XBRL DOCUMENT
    <TEXT>
    Document 14 - file: MetaLinks.json

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>ZIP
    <SEQUENCE>15
    <FILENAME>0000875045-21-000049-xbrl.zip
    <DESCRIPTION>IDEA: XBRL DOCUMENT
    <TEXT>
    Document 15 - file: 0000875045-21-000049-xbrl.zip

    </DOCUMENT>
    </SEC-DOCUMENT>