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Equity (Details Textual) (USD $) | 3 Months Ended |
--- | --- | ---
Mar. 31, 2014 | Mar. 31, 2013 | Feb. 28, 2011
Equity (Textual) [Abstract] |  |  |
Equity increase during the period | $ 520,200,000 |  |
Net income attributable to Biogen Idec Inc | 479,956,000 | 426,749,000 |
Adjustments to Additional Paid in Capital, Share-based Compensation and Exercise of Stock Options | 31,100,000 |  |
Purchase of treasury stock, value | $ 0 | $ 41,023,000 |
Common stock shares authorized for repurchase | 4,200,000 |  | 20,000,000.0
Repurchase of common stock, shares |  | 300,000 |

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