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Equity (Details Textual) (USD $) | 3 Months Ended | 6 Months Ended |
--- | --- | --- | ---
Jun. 30, 2014 | Jun. 30, 2013 | Jun. 30, 2014 | Jun. 30, 2013 | Feb. 28, 2011
Equity (Textual) [Abstract] |  |  |  |  |
Equity increase during the period |  |  | $ 944,400,000 |  |
Net income attributable to Biogen Idec Inc | 714,506,000 | 490,670,000 | 1,194,462,000 | 917,419,000 |
Adjustments to additional paid in capital, share-based compensation and exercise of stock options |  |  | 82,400,000 |  |
Common stock shares authorized for repurchase | 3,000,000 |  | 3,000,000 |  | 20,000,000.0
Repurchase of common stock, shares |  |  | 1,200,000 | 300,000 |
Purchase of treasury stock, value |  |  | $ 336,905,000 | $ 41,023,000 |

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