Earnings per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Numerator:
Net income attributable to Biogen Idec Inc	$ 487,616	$ 398,400	$ 1,405,035	$ 1,087,906
Denominator:
Weighted average number of common shares outstanding	237,070,000	236,474,000	237,131,000	238,331,000
Effect of dilutive securities:
Dilutive potential common shares	1,200,000	1,600,000	1,400,000	1,800,000
Shares used in calculating diluted earnings per share	238,349,000	238,125,000	238,508,000	240,137,000
Stock options and employee stock purchase plan [Member]
Effect of dilutive securities:
Stock options and employee stock purchase plan	200,000	400,000	400,000	500,000
Time-vested restricted stock units [Member]
Effect of dilutive securities:
Stock options and employee stock purchase plan	700,000	900,000	700,000	1,000,000
Market stock units [Member]
Effect of dilutive securities:
Stock options and employee stock purchase plan	300,000	300,000	300,000	300,000