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Equity (Details Textual) - USD ($) shares in Millions, $ in Millions | 3 Months Ended |  |
--- | --- | --- | ---
Mar. 31, 2018 | Mar. 31, 2017 | Sep. 30, 2016 | Mar. 31, 2011
Class of Stock [Line Items] |  |  |  |
Payments for repurchase of common stock | $ 250.0 | $ 583.6 |  |
Equity increase during the period | 1,400.0 |  |  |
Net income attributable to Biogen Inc. | 1,172.9 | $ 747.6 |  |
2016 Share Repurchase Program |  |  |  |
Class of Stock [Line Items] |  |  |  |
Authorized amount of share repurchases under the 2016 Share Repurchase Program | $ 2,800.0 |  | $ 5,000.0 |
Repurchase and retire of common stock, shares | 0.9 | 0.8 |  |
Payments for repurchase of common stock | $ 250.0 | $ 218.2 |  |
2011 Share Repurchase Program |  |  |  |
Class of Stock [Line Items] |  |  |  |
Common stock shares authorized for repurchased |  |  |  | 20.0
Repurchase of common stock, shares |  | 1.2 |  |
Payments for repurchase of common stock |  | $ 365.4 |  |

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