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Equity (Details Textual) (USD $) | 3 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Sep. 30, 2014 | Sep. 30, 2013 | Sep. 30, 2014 | Sep. 30, 2013 | Feb. 28, 2011
Equity (Textual) [Abstract] |  |  |  |  |
Equity increase during the period |  |  | $ 1,810,000,000 |  |
Net income attributable to Biogen Idec Inc | 856,858,000 | 487,616,000 | 2,051,320,000 | 1,405,035,000 |
Adjustments to additional paid in capital, share-based compensation and exercise of stock options |  |  | 126,400,000 |  |
Common stock shares authorized for repurchase | 2,900,000 |  | 2,900,000 |  | 20,000,000.0
Repurchase of common stock, shares |  |  | 1,200,000 | 2,000,000 |
Purchase of treasury stock, value |  |  | $ 359,981,000 | $ 400,308,000 |

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