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Equity (Details Textual) - USD ($) shares in Millions, $ in Millions | 3 Months Ended |  |
--- | --- | --- | ---
Mar. 31, 2017 | Mar. 31, 2016 | Sep. 30, 2016 | Mar. 31, 2011
Class of Stock [Line Items] |  |  |  |
Payments for repurchase of common stock | $ 583.6 | $ 0.0 |  |
Equity increase during the period | 661.2 |  |  |
Adjustment to retained earnings reflecting spin-off | 852.8 |  |  |
Net income attributable to Biogen Inc. | $ 747.6 | $ 970.9 |  |
2016 Share Repurchase Program [Member] |  |  |  |
Class of Stock [Line Items] |  |  |  |
Authorized amount of share repurchases under the 2015 Share Repurchase Program |  |  | $ 5,000.0 |
Repurchase of common stock, shares | 0.8 |  |  |
Payments for repurchase of common stock | $ 218.2 |  |  |
2016 Share Repurchase Program Plan To Date [Member] |  |  |  |
Class of Stock [Line Items] |  |  |  |
Repurchase of common stock, shares | 4.1 |  |  |
Payments for repurchase of common stock | $ 1,200.0 |  |  |
2011 Share Repurchase Program [Member] |  |  |  |
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 |  |  |

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