Earnings per Share (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Numerator:
Net income attributable to Biogen Idec Inc	$ 351,826,000	$ 254,115,000	$ 934,190,000	$ 764,995,000
Adjustment for net income allocable to preferred stock	0	(500,000)	(600,000)	(1,500,000)
Net income used in calculating basic and diluted earnings per share	351,800,000	253,600,000	933,600,000	763,500,000
Denominator:
Weighted average number of common shares outstanding	242,883,000	239,864,000	242,266,000	256,586,000
Effect of dilutive securities:
Dilutive potential common shares	2,500,000	2,400,000	2,800,000	2,300,000
Shares used in calculating diluted earnings per share	245,366,000	242,313,000	245,140,000	258,906,000
Numerator:
Net income allocable to preferred stock	$ 0	$ 500,000	$ 600,000	$ 1,500,000
Denominator:
Total	0	6,200,000	100,000	6,400,000
Stock options and employee stock purchase plan [Member]
Effect of dilutive securities:
Stock options and employee stock purchase plan	700,000	900,000	1,100,000	900,000
Denominator:
Total	0	4,500,000	0	4,900,000
Market stock units [Member]
Effect of dilutive securities:
Stock options and employee stock purchase plan	200,000		200,000
Denominator:
Total	0		0
Time Vested Restricted Stock Units [Member]
Effect of dilutive securities:
Stock options and employee stock purchase plan	1,600,000	1,500,000	1,500,000	1,400,000
Denominator:
Total	0	1,200,000	0	1,000,000
Performance vested restricted stock units settled in shares [Member]
Effect of dilutive securities:
Stock options and employee stock purchase plan	0		0
Denominator:
Total	0		0
Convertible preferred stock [Member]
Denominator:
Total	0	500,000	100,000	500,000