v2.4.0.6

Equity (Details Textual) (USD $) Share data in Millions, unless otherwise specified | 3 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Sep. 30, 2012 | Sep. 30, 2011 | Sep. 30, 2012 | Sep. 30, 2011 | Feb. 28, 2011
Equity (Textual) [Abstract] |  |  |  |  |
Equity increase (decrease) during the period |  |  | $ 235,000,000 |  |
Net income attributable to Biogen Idec Inc | 398,400,000 | 351,826,000 | 1,087,906,000 | 934,190,000 |
Increase in additional paid in capital, share based compensation |  |  | 134,000,000 |  |
Repurchases of common stock |  |  | 963,171,000 | 386,575,000 |
Common stock shares authorized for repurchase | 6.3 |  | 6.3 |  | 20.0
Repurchase of common stock, shares | 0.4 |  | 7.7 | 5.0 |
Repurchase of common stock, value | $ 53,200,000 |  | $ 963,200,000 | $ 386,600,000 |

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