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