Earnings per Share (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Numerator:
Net income attributable to Biogen Idec Inc	$ 398,400,000	$ 351,826,000	$ 1,087,906,000	$ 934,190,000
Adjustment for net income allocable to preferred stock	0	0	0	(600,000)
Net income used in calculating basic and diluted earnings per share	$ 398,400,000	$ 351,800,000	$ 1,087,900,000	$ 933,600,000
Denominator:
Weighted average number of common shares outstanding	236,474,000	242,883,000	238,331,000	242,266,000
Effect of dilutive securities:
Dilutive potential common shares	1,600,000	2,500,000	1,800,000	2,800,000
Shares used in calculating diluted earnings per share	238,125,000	245,366,000	240,137,000	245,140,000
Stock options and employee stock purchase plan [Member]
Effect of dilutive securities:
Stock options and employee stock purchase plan	400,000	700,000	500,000	1,100,000
Time-vested restricted stock units [Member]
Effect of dilutive securities:
Stock options and employee stock purchase plan	900,000	1,600,000	1,000,000	1,500,000
Market stock units [Member]
Effect of dilutive securities:
Stock options and employee stock purchase plan	300,000	200,000	300,000	200,000