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Pay vs Performance Disclosure - USD ($) | 12 Months Ended
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Dec. 31, 2022 | Dec. 31, 2021 | Dec. 31, 2020
Pay vs Performance Disclosure [Table] |  |  |
Pay vs Performance [Table Text Block] | Pay versus Performance Comparison As discussed in our CD&A above, our CMDC has implemented executive compensation programs designed to link a substantial part of our NEOs compensation to financial, strategic, and market-based measures and align realizable compensation with stockholder return. The following table sets forth additional compensation information for our NEOs, calculated in accordance with SEC regulations, for fiscal years 2022, 2021 and 2020. Pay versus Performance Tabular Disclosure: The table below contains information about the relationship between compensation actually paid, as computed in accordance with SEC rules, to our CEO and non-CEO NEOs as a group ,and our financial performance for the three years 2020 – 2022. The cumulative Total Stockholder Return depicts a hypothetical $100 investment in our common stock on December 31, 2019, and shows the value of that investment over time for each calendar year. A hypothetical $100 investment in the NASDAQ Biotech Index using the same methodology is shown for comparison. The CMDC did not consider the pay versus performance disclosure below in making its pay decisions for any of the fiscal years shown. In addition, the average non-CEO NEO incumbents differ year to year and may comprise different roles with different individual and market-based considerations impacting compensation decisions. Comparing the summary compensation table total average and compensation actually paid average for the non-CEO NEOs year over year does not accurately depict changes in these values for the same individuals.
Fiscal Year |  | SCT Total for CEO: C. Viehbacher (1) |  |  | SCT Total for Former CEO: M. Vounatsos (1) |  |  | Compensation Actually Paid to CEO (2) |  |  | Compensation Actually Paid to Former CEO (2) |  |  | Average SCT Total for Other NEOs |  |  | Average Compensation Actually Paid for Other NEOs (2)(3) |  |  | Value of $100 Initial Fixed Investment Based on TSR |  |  | Value of $100 Initial Fixed Investment Based on NASDAQ Biotech Index TSR (4) |  |  | Net Income ($M) |  |  | Company Selected Measure: Revenue (5) |
2022 |  | $ | 30,488,593 |  |  | $ | 26,625,221 |  |  | $ | 27,551,849 |  |  | $ | 5,898,711 |  |  | $ | 5,878,461 |  |  | $ | 4,969,471 |  |  | $ | 93.32 |  |  | $ | 111.27 |  |  | $ | 3,047 |  |  | $ | 10,326M |
2021 |  |  | — |  |  | $ | 17,689,665 |  |  |  | — |  |  | $ | 15,211,025 |  |  | $ | 5,761,560 |  |  | $ | 5,081,941 |  |  | $ | 80.85 |  |  | $ | 124.89 |  |  | $ | 1,556 |  |  | $ | 11,054M |
2020 |  |  | — |  |  | $ | 18,659,829 |  |  |  | — |  |  | $ | 8,578,367 |  |  | $ | 6,525,375 |  |  | $ | 4,363,675 |  |  | $ | 82.52 |  |  | $ | 125.69 |  |  | $ | 4,001 |  |  | $ | 13,952M |
Notes to Pay Vs. Performance Table
(1) | The dollar amounts reported are the amounts of total compensation reported for our CEO, Mr. Viehbacher, and our former CEO, Mr. Vounatsos in the Summary Compensation Table for fiscal years 2022, 2021 and 2020. Mr. Vounatsos served as CEO for each of the full fiscal years 2021 and 2020, while Mr. Viehbacher and Mr. Vounatsos both served as CEO in fiscal year 2022.
(2) | The dollar amounts reported represent the amount of “compensation actually paid”, as computed in accordance with SEC rules. The dollar amounts do not reflect the actual amounts of compensation paid to our CEO and other NEOs during the applicable year, but also include (i) the year-end fair value of equity awards granted during the reported year, (ii) the change in fair value of equity awards that were unvested at the end of the prior year, measured through the date the awards vested or were forfeited, or through the end of the reported fiscal year.
(3) | For 2022, reflects compensation information for our NEOs, other than our CEO, as described in the CD&A of this proxy statement. For 2021, reflects compensation information for Mr. McDonnell, Ms. Alexander, Mr. Guindo and Alfred Sandrock, our former EVP of R&D. For 2020, reflects compensation information for Mr. McDonnell, Ms. Alexander, Mr. Guindo, Dr. Sandrock and Jeffrey Capello, our former CFO.
(4) | Reflects cumulative total stockholder return of the NASDAQ Biotechnology Index (NBI) as of December 31, 2022. The NBI is the peer group used by BIIB for the purposes of item 201(e) of Regulation S-K under the Exchange Act in the Company’s Annual Report on Form 10-K for the year ended December 31, 2022.
(5) | Revenue was adjusted to neutralize the effects of foreign exchange rate fluctuations.
The following tables set forth the adjustments made during each year in the PVP table to arrive at “compensation actually paid” to our Principal Executive Officer during each year.
Fiscal Year |  | Summary Compensation Table Total for CEO |  |  | Summary Compensation Table Total for former CEO |  |  | Reported Value of Equity Awards for CEO (1) |  |  | Reported Value of Equity Awards for former CEO (1) |  |  | Equity Award Adjustments for CEO (2) |  |  | Equity Award Adjustments for former CEO (2) |  |  | Compensation Actually Paid to CEO |  |  | Compensation Actually Paid to former CEO |
2022 |  | $ | 30,488,593 |  |  | $ | 26,625,221 |  |  | $ | 30,000,655 |  |  | $ | 15,389,732 |  |  | $ | 27,063,911 |  |  | -$ | 5,336,778 |  |  | $ | 27,551,849 |  |  | $ | 5,898,711 |
2021 |  |  | — |  |  | $ | 17,689,665 |  |  |  | — |  |  | $ | 14,084,314 |  |  |  | — |  |  | $ | 11,605,674 |  |  |  | — |  |  | $ | 15,211,025 |
2020 |  |  | — |  |  | $ | 18,659,829 |  |  |  | — |  |  | $ | 13,887,064 |  |  |  | — |  |  | $ | 3,805,602 |  |  |  | — |  |  | $ | 8,578,367 |
(1) | Represents the grant date fair value of equity awards to our CEO and former CEO, as reported in the “Stock Awards” and “Option Awards” columns in the Summary Compensation Table for each applicable year.
(2) | Represents the year-over-year change in the fair value of equity awards to our CEO and former CEO, as itemized in the table below. No awards vested in the year they were granted.
|  | 2022 |  |  |  |  |  |  |
Fair Value of Equity Awards for CEO and former CEO |  | Mr. Viehbacher |  |  | Mr. Vounatsos |  |  | 2021 |  |  | 2020 |
As of year-end for awards granted during the year |  | $ | 27,063,911 |  |  | $ | 1,996,870 |  |  | $ | 10,985,776 |  |  | $ | 8,893,002 |
Year-over-year increase or decrease of unvested awards granted in prior years |  | $ | 0 |  |  | -$ | 6,247,880 |  |  | -$ | 687,744 |  |  | -$ | 3,618,885 |
Increase or decrease from prior fiscal year-end for awards that vested during the year |  | $ | 0 |  |  | -$ | 1,085,768 |  |  | $ | 1,307,643 |  |  | -$ | 1,468,515 |
Total Equity Award Adjustments |  | $ | 27,063,911 |  |  | -$ | 5,336,778 |  |  | $ | 11,605,674 |  |  | $ | 3,805,602 |
The following tables set forth the adjustments made during each year in the PVP table to arrive at “compensation actually paid” to our non-CEO NEOs during each year.
Fiscal Year |  | Average Summary Compensation Table Total for Non-CEO NEOs |  |  | Average Reported Value of Equity Awards for non-CEO NEOs (1) |  |  | Average Equity Award Adjustments for non-CEO NEOs (2) |  |  | Average Compensation Actually Paid to non-CEO NEOs |
2022 |  | $ | 5,878,461 |  |  | $ | 4,223,226 |  |  | $ | 3,314,236 |  |  | $ | 4,969,471 |
2021 |  | $ | 5,761,560 |  |  | $ | 4,105,942 |  |  | $ | 3,426,323 |  |  | $ | 5,081,941 |
2020 |  | $ | 6,525,375 |  |  | $ | 4,158,006 |  |  | $ | 1,996,306 |  |  | $ | 4,363,675 |
(1) | Represents the average of the grant date fair value of the equity awards to our named executive officers (other than our CEO and former CEO), as reported in the “Stock Awards” column in the SCT for each applicable year.
(2) | Represents the average of the year-over-year change in fair value of equity awards to our named executive officers (other than our CEO and former CEO), as itemized in the table below. No awards vested in the year they were granted.
Average Fair Value of Equity Awards for non-CEO NEOs |  | 2022 |  |  | 2021 |  |  | 2020 |
As of year-end for awards granted during the year |  | $ | 3,809,706 |  |  | $ | 3,295,987 |  |  | $ | 2,904,843 |
Year-over-year increase or decrease of unvested awards granted in prior years |  | -$ | 244,173 |  |  | -$ | 156,359 |  |  | -$ | 741,018 |
Increase or decrease from prior fiscal year-end for awards that vested during the year |  | -$ | 251,296 |  |  | $ | 286,695 |  |  | -$ | 167,520 |
Total Equity Award Adjustments |  | $ | 3,314,236 |  |  | $ | 3,426,323 |  |  | $ | 1,996,306 |
Company Selected Measure Name | Revenue |  |
Named Executive Officers, Footnote [Text Block] | For 2022, reflects compensation information for our NEOs, other than our CEO, as described in the CD&A of this proxy statement. For 2021, reflects compensation information for Mr. McDonnell, Ms. Alexander, Mr. Guindo and Alfred Sandrock, our former EVP of R&D. For 2020, reflects compensation information for Mr. McDonnell, Ms. Alexander, Mr. Guindo, Dr. Sandrock and Jeffrey Capello, our former CFO |  |
Peer Group Issuers, Footnote [Text Block] | Reflects cumulative total stockholder return of the NASDAQ Biotechnology Index (NBI) as of December 31, 2022. The NBI is the peer group used by BIIB for the purposes of item 201(e) of Regulation S-K under the Exchange Act in the Company’s Annual Report on Form 10-K for the year ended December 31, 2022. |  |
Adjustment To PEO Compensation, Footnote [Text Block] | The following tables set forth the adjustments made during each year in the PVP table to arrive at “compensation actually paid” to our Principal Executive Officer during each year.
Fiscal Year |  | Summary Compensation Table Total for CEO |  |  | Summary Compensation Table Total for former CEO |  |  | Reported Value of Equity Awards for CEO (1) |  |  | Reported Value of Equity Awards for former CEO (1) |  |  | Equity Award Adjustments for CEO (2) |  |  | Equity Award Adjustments for former CEO (2) |  |  | Compensation Actually Paid to CEO |  |  | Compensation Actually Paid to former CEO |
2022 |  | $ | 30,488,593 |  |  | $ | 26,625,221 |  |  | $ | 30,000,655 |  |  | $ | 15,389,732 |  |  | $ | 27,063,911 |  |  | -$ | 5,336,778 |  |  | $ | 27,551,849 |  |  | $ | 5,898,711 |
2021 |  |  | — |  |  | $ | 17,689,665 |  |  |  | — |  |  | $ | 14,084,314 |  |  |  | — |  |  | $ | 11,605,674 |  |  |  | — |  |  | $ | 15,211,025 |
2020 |  |  | — |  |  | $ | 18,659,829 |  |  |  | — |  |  | $ | 13,887,064 |  |  |  | — |  |  | $ | 3,805,602 |  |  |  | — |  |  | $ | 8,578,367 |
(1) | Represents the grant date fair value of equity awards to our CEO and former CEO, as reported in the “Stock Awards” and “Option Awards” columns in the Summary Compensation Table for each applicable year.
(2) | Represents the year-over-year change in the fair value of equity awards to our CEO and former CEO, as itemized in the table below. No awards vested in the year they were granted.
|  | 2022 |  |  |  |  |  |  |
Fair Value of Equity Awards for CEO and former CEO |  | Mr. Viehbacher |  |  | Mr. Vounatsos |  |  | 2021 |  |  | 2020 |
As of year-end for awards granted during the year |  | $ | 27,063,911 |  |  | $ | 1,996,870 |  |  | $ | 10,985,776 |  |  | $ | 8,893,002 |
Year-over-year increase or decrease of unvested awards granted in prior years |  | $ | 0 |  |  | -$ | 6,247,880 |  |  | -$ | 687,744 |  |  | -$ | 3,618,885 |
Increase or decrease from prior fiscal year-end for awards that vested during the year |  | $ | 0 |  |  | -$ | 1,085,768 |  |  | $ | 1,307,643 |  |  | -$ | 1,468,515 |
Total Equity Award Adjustments |  | $ | 27,063,911 |  |  | -$ | 5,336,778 |  |  | $ | 11,605,674 |  |  | $ | 3,805,602 |
Non-PEO NEO Average Total Compensation Amount | $ 5,878,461 | $ 5,761,560 | $ 6,525,375
Non-PEO NEO Average Compensation Actually Paid Amount | $ 4,969,471 | 5,081,941 | 4,363,675
Adjustment to Non-PEO NEO Compensation Footnote [Text Block] | The following tables set forth the adjustments made during each year in the PVP table to arrive at “compensation actually paid” to our non-CEO NEOs during each year.
Fiscal Year |  | Average Summary Compensation Table Total for Non-CEO NEOs |  |  | Average Reported Value of Equity Awards for non-CEO NEOs (1) |  |  | Average Equity Award Adjustments for non-CEO NEOs (2) |  |  | Average Compensation Actually Paid to non-CEO NEOs |
2022 |  | $ | 5,878,461 |  |  | $ | 4,223,226 |  |  | $ | 3,314,236 |  |  | $ | 4,969,471 |
2021 |  | $ | 5,761,560 |  |  | $ | 4,105,942 |  |  | $ | 3,426,323 |  |  | $ | 5,081,941 |
2020 |  | $ | 6,525,375 |  |  | $ | 4,158,006 |  |  | $ | 1,996,306 |  |  | $ | 4,363,675 |
(1) | Represents the average of the grant date fair value of the equity awards to our named executive officers (other than our CEO and former CEO), as reported in the “Stock Awards” column in the SCT for each applicable year.
(2) | Represents the average of the year-over-year change in fair value of equity awards to our named executive officers (other than our CEO and former CEO), as itemized in the table below. No awards vested in the year they were granted.
Average Fair Value of Equity Awards for non-CEO NEOs |  | 2022 |  |  | 2021 |  |  | 2020 |
As of year-end for awards granted during the year |  | $ | 3,809,706 |  |  | $ | 3,295,987 |  |  | $ | 2,904,843 |
Year-over-year increase or decrease of unvested awards granted in prior years |  | -$ | 244,173 |  |  | -$ | 156,359 |  |  | -$ | 741,018 |
Increase or decrease from prior fiscal year-end for awards that vested during the year |  | -$ | 251,296 |  |  | $ | 286,695 |  |  | -$ | 167,520 |
Total Equity Award Adjustments |  | $ | 3,314,236 |  |  | $ | 3,426,323 |  |  | $ | 1,996,306 |
Compensation Actually Paid vs. Total Shareholder Return [Text Block] |  |  |
Compensation Actually Paid vs. Company Selected Measure [Text Block] |  |  |
Total Shareholder Return Vs Peer Group [Text Block] |  |  |
Tabular List [Table Text Block] |
Most Important Performance Measures
Revenue
Total Shareholder Return
Non-GAAP Earnings Per Share
Pipeline Milestones
Total Shareholder Return Amount | $ 93.32 | 80.85 | 82.52
Peer Group Total Shareholder Return Amount | 111.27 | 124.89 | 125.69
Net Income (Loss) | $ 3,047,000,000 | $ 1,556,000,000 | $ 4,001,000,000
Company Selected Measure Amount | 10,326,000,000 | 11,054,000,000 | 13,952,000,000
Measure [Axis]: 1 |  |  |
Pay vs Performance Disclosure [Table] |  |  |
Measure Name | Revenue |  |
Measure [Axis]: 2 |  |  |
Pay vs Performance Disclosure [Table] |  |  |
Measure Name | Total Shareholder Return |  |
Measure [Axis]: 3 |  |  |
Pay vs Performance Disclosure [Table] |  |  |
Compensation Actually Paid vs. Other Measure [Text Block] |  |  |
Measure Name | Non-GAAP Earnings Per Share |  |
Measure [Axis]: 4 |  |  |
Pay vs Performance Disclosure [Table] |  |  |
Measure Name | Pipeline Milestones |  |
Mr.Viehbacher [Member] |  |  |
Pay vs Performance Disclosure [Table] |  |  |
PEO Total Compensation Amount | $ 30,488,593 | $ 0 | $ 0
PEO Actually Paid Compensation Amount | $ 27,551,849 | 0 | 0
PEO Name | Mr. Viehbacher |  |
Mr.Vounatsos [Member] |  |  |
Pay vs Performance Disclosure [Table] |  |  |
PEO Total Compensation Amount | $ 26,625,221 | 17,689,665 | 18,659,829
PEO Actually Paid Compensation Amount | $ 5,898,711 | 15,211,025 | 8,578,367
PEO Name | Mr. Vounatsos |  |
PEO [Member] | Mr.Viehbacher [Member] | Stock Awards And Option Awards [Member] |  |  |
Pay vs Performance Disclosure [Table] |  |  |
Adjustment to Compensation Amount | $ 30,000,655 | 0 | 0
PEO [Member] | Mr.Viehbacher [Member] | YearEnd For Awards Granted During The Year [Member] |  |  |
Pay vs Performance Disclosure [Table] |  |  |
Adjustment to Compensation Amount | 27,063,911 |  |
PEO [Member] | Mr.Viehbacher [Member] | YearOverYear Increase Or Decrease Of Unvested Awards Granted In Prior Years [Member] |  |  |
Pay vs Performance Disclosure [Table] |  |  |
Adjustment to Compensation Amount | 0 |  |
PEO [Member] | Mr.Viehbacher [Member] | Prior Fiscal YearEnd For Awards That Vested During The Year [Member] |  |  |
Pay vs Performance Disclosure [Table] |  |  |
Adjustment to Compensation Amount | 0 |  |
PEO [Member] | Mr.Viehbacher [Member] | Total Equity Award Adjustments [Member] |  |  |
Pay vs Performance Disclosure [Table] |  |  |
Adjustment to Compensation Amount | 27,063,911 |  |
PEO [Member] | Mr.Viehbacher [Member] | YearOverYear Change In Fair Value Of Equity Awards [Member] |  |  |
Pay vs Performance Disclosure [Table] |  |  |
Adjustment to Compensation Amount | 27,063,911 | 0 | 0
PEO [Member] | Mr.Vounatsos [Member] | Stock Awards And Option Awards [Member] |  |  |
Pay vs Performance Disclosure [Table] |  |  |
Adjustment to Compensation Amount | 15,389,732 | 14,084,314 | 13,887,064
PEO [Member] | Mr.Vounatsos [Member] | YearEnd For Awards Granted During The Year [Member] |  |  |
Pay vs Performance Disclosure [Table] |  |  |
Adjustment to Compensation Amount | 1,996,870 | 10,985,776 | 8,893,002
PEO [Member] | Mr.Vounatsos [Member] | YearOverYear Increase Or Decrease Of Unvested Awards Granted In Prior Years [Member] |  |  |
Pay vs Performance Disclosure [Table] |  |  |
Adjustment to Compensation Amount | (6,247,880) | (687,744) | (3,618,885)
PEO [Member] | Mr.Vounatsos [Member] | Prior Fiscal YearEnd For Awards That Vested During The Year [Member] |  |  |
Pay vs Performance Disclosure [Table] |  |  |
Adjustment to Compensation Amount | (1,085,768) | 1,307,643 | (1,468,515)
PEO [Member] | Mr.Vounatsos [Member] | Total Equity Award Adjustments [Member] |  |  |
Pay vs Performance Disclosure [Table] |  |  |
Adjustment to Compensation Amount | (5,336,778) | 11,605,674 | 3,805,602
PEO [Member] | Mr.Vounatsos [Member] | YearOverYear Change In Fair Value Of Equity Awards [Member] |  |  |
Pay vs Performance Disclosure [Table] |  |  |
Adjustment to Compensation Amount | (5,336,778) | 11,605,674 | 3,805,602
Non-PEO NEO [Member] | Stock Awards And Option Awards [Member] |  |  |
Pay vs Performance Disclosure [Table] |  |  |
Adjustment to Compensation Amount | 4,223,226 | 4,105,942 | 4,158,006
Non-PEO NEO [Member] | YearEnd For Awards Granted During The Year [Member] |  |  |
Pay vs Performance Disclosure [Table] |  |  |
Adjustment to Compensation Amount | 3,809,706 | 3,295,987 | 2,904,843
Non-PEO NEO [Member] | YearOverYear Increase Or Decrease Of Unvested Awards Granted In Prior Years [Member] |  |  |
Pay vs Performance Disclosure [Table] |  |  |
Adjustment to Compensation Amount | (244,173) | (156,359) | (741,018)
Non-PEO NEO [Member] | Prior Fiscal YearEnd For Awards That Vested During The Year [Member] |  |  |
Pay vs Performance Disclosure [Table] |  |  |
Adjustment to Compensation Amount | (251,296) | 286,695 | (167,520)
Non-PEO NEO [Member] | Total Equity Award Adjustments [Member] |  |  |
Pay vs Performance Disclosure [Table] |  |  |
Adjustment to Compensation Amount | 3,314,236 | 3,426,323 | 1,996,306
Non-PEO NEO [Member] | YearOverYear Change In Fair Value Of Equity Awards [Member] |  |  |
Pay vs Performance Disclosure [Table] |  |  |
Adjustment to Compensation Amount | $ 3,314,236 | $ 3,426,323 | $ 1,996,306

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