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Earnings per Share (Details) - USD ($) shares in Millions, $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2017 | Sep. 30, 2017 | Jun. 30, 2017 | Mar. 31, 2017 | Dec. 31, 2016 | Sep. 30, 2016 | Jun. 30, 2016 | Mar. 31, 2016 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Numerator: |  |  |  |  |  |  |  |  |  |  |
Net income attributable to Biogen Idec Inc | $ (297.4) | $ 1,226.1 | $ 862.8 | $ 747.6 | $ 649.2 | $ 1,032.9 | $ 1,049.8 | $ 970.9 | $ 2,539.1 | $ 3,702.8 | $ 3,547.0
Denominator: |  |  |  |  |  |  |  |  |  |  |
Weighted average number of common shares outstanding | 211.5 | 211.4 | 211.9 | 215.6 | 216.6 | 218.9 | 219.1 | 218.9 | 212.6 | 218.4 | 230.7
Effect of dilutive securities: |  |  |  |  |  |  |  |  |  |  |
Dilutive potential common shares |  |  |  |  |  |  |  |  | 0.4 | 0.4 | 0.5
Shares used in calculating diluted earnings per share | 212.0 | 211.8 | 212.2 | 215.9 | 217.0 | 219.4 | 219.4 | 219.3 | 213.0 | 218.8 | 231.2
Earnings per share (Textual) |  |  |  |  |  |  |  |  |  |  |
Repurchase of common stock |  |  |  |  |  |  |  |  | 3.7 | 0.7 | 4.6
Stock options and employee stock purchase plan |  |  |  |  |  |  |  |  |  |  |
Effect of dilutive securities: |  |  |  |  |  |  |  |  |  |  |
Stock options and employee stock purchase plan |  |  |  |  |  |  |  |  | 0.1 | 0.1 | 0.1
Time-vested restricted stock units |  |  |  |  |  |  |  |  |  |  |
Effect of dilutive securities: |  |  |  |  |  |  |  |  |  |  |
Stock options and employee stock purchase plan |  |  |  |  |  |  |  |  | 0.2 | 0.2 | 0.3
Market stock units |  |  |  |  |  |  |  |  |  |  |
Effect of dilutive securities: |  |  |  |  |  |  |  |  |  |  |
Stock options and employee stock purchase plan |  |  |  |  |  |  |  |  | 0.1 | 0.1 | 0.1

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