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Earnings per share.

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Numerator.

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Additional shares included in the calculation of diluted EPS as a result of the potentially dilutive effect of share based payment arrangements using the treasury stock method.

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The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

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Earnings per Share (Details) - USD ($) shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Numerator:
Net income attributable to Biogen Idec Inc	$ 946.8	$ 1,444.4	$ 866.6	$ 1,172.9	$ (297.4)	$ 1,226.1	$ 862.8	$ 747.6	$ 4,430.7	$ 2,539.1	$ 3,702.8
Denominator:
Weighted average number of common shares outstanding	199.8	201.4	207.1	211.4	211.5	211.4	211.9	215.6	204.9	212.6	218.4
Effect of dilutive securities:
Dilutive potential common shares									0.4	0.4	0.4
Shares used in calculating diluted earnings per share	200.3	201.9	207.3	211.7	212.0	211.8	212.2	215.9	205.3	213.0	218.8
Earnings per share (Textual)
Repurchase of common stock									14.8	3.7	0.7
Stock options and employee stock purchase plan
Effect of dilutive securities:
Stock options and employee stock purchase plan									0.0	0.1	0.1
Time-vested restricted stock units
Effect of dilutive securities:
Stock options and employee stock purchase plan									0.3	0.2	0.2
Market stock units
Effect of dilutive securities:
Stock options and employee stock purchase plan									0.1	0.1	0.1
Performance stock units settled in stock
Effect of dilutive securities:
Stock options and employee stock purchase plan									0.0	0.0	0.0