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Equity (Details Textual) - USD ($) shares in Millions, $ in Millions | 3 Months Ended | 6 Months Ended |  |  |
--- | --- | --- | --- | --- | ---
Jun. 30, 2018 | Jun. 30, 2017 | Jun. 30, 2018 | Jun. 30, 2017 | Jan. 01, 2018 | Sep. 30, 2016 | Mar. 31, 2011
Class of Stock [Line Items] |  |  |  |  |  |  |
Payments for repurchase of common stock |  |  | $ 3,000.0 | $ 1,365.4 |  |  |
Equity increase during the period |  |  | 344.4 |  |  |  |
Net income attributable to Biogen Inc. | $ 866.6 | $ 862.8 | $ 2,039.5 | $ 1,610.4 |  |  |
2016 Share Repurchase Program |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |
Authorized amount of share repurchases under the 2016 Share Repurchase Program |  |  |  |  |  | $ 5,000.0 |
Repurchase and retire of common stock, shares | 9.6 | 2.9 | 10.5 | 3.7 |  |  |
Payments for repurchase of common stock | $ 2,750.0 | $ 781.8 | $ 3,000.0 | $ 1,000.0 |  |  |
2011 Share Repurchase Program |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |
Common stock shares authorized for repurchased |  |  |  |  |  |  | 20.0
Repurchase of common stock, shares |  |  |  | 1.3 |  |  |
Payments for repurchase of common stock |  |  |  | $ 365.4 |  |  |
Accounting Standards Update 2016-16 | Retained earnings |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |
Cumulative effect of new accounting principle |  |  |  |  | $ 500.0 |  |

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