Earnings per Share (Details) - USD ($) shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Numerator:
Net income attributable to Biogen Idec Inc	$ 831.6	$ 965.6	$ 927.3	$ 822.5	$ 883.5	$ 856.9	$ 714.5	$ 480.0	$ 3,547.0	$ 2,934.8	$ 1,862.3
Denominator:
Weighted average number of common shares outstanding	220.4	232.2	235.3	235.0	235.5	236.2	236.7	236.8	230.7	236.4	236.9
Effect of dilutive securities:
Dilutive potential common shares									0.5	0.8	1.4
Shares used in calculating diluted earnings per share	220.8	232.6	235.7	235.6	236.3	237.0	237.4	237.8	231.2	237.2	238.3
Earnings per share (Textual)
Repurchase of common stock									4.6	1.0	0.9
Stock options and employee stock purchase plan
Effect of dilutive securities:
Stock options and employee stock purchase plan									0.1	0.1	0.3
Time-vested restricted stock units
Effect of dilutive securities:
Stock options and employee stock purchase plan									0.3	0.5	0.8
Market stock units
Effect of dilutive securities:
Stock options and employee stock purchase plan									0.1	0.2	0.3