Earnings per Share (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Numerator:
Net income attributable to Biogen Idec Inc	$ 288,034,000	$ 293,438,000	$ 582,364,000	$ 510,880,000
Adjustment for net income allocable to preferred stock	0	(600,000)	(500,000)	(900,000)
Net income used in calculating basic and diluted earnings per share	288,000,000	292,800,000	581,900,000	510,000,000
Denominator:
Weighted average number of common shares outstanding	242,375,000	259,938,000	241,932,000	265,018,000
Effect of dilutive securities:
Stock options and employee stock purchase plan	0			0
Dilutive potential common shares	2,600,000	1,800,000	3,000,000	2,300,000
Shares used in calculating diluted earnings per share	244,966,000	261,658,000	244,899,000	267,272,000
Numerator:
Net income allocable to preferred stock	$ 0	$ 600,000	$ 500,000	$ 900,000
Denominator:
Total	0	7,200,000	200,000	6,600,000
Stock options and employee stock purchase plan [Member]
Effect of dilutive securities:
Stock options and employee stock purchase plan	1,000,000	800,000	1,300,000	900,000
Denominator:
Total	0	5,300,000		5,100,000
Market stock units [Member]
Effect of dilutive securities:
Stock options and employee stock purchase plan	200,000		200,000
Denominator:
Total	0	0	0	0
Time Vested Restricted Stock Units [Member]
Effect of dilutive securities:
Stock options and employee stock purchase plan	1,400,000	1,000,000	1,500,000	1,400,000
Denominator:
Total		1,400,000		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	0	0
Convertible preferred stock [Member]
Denominator:
Total	0	500,000	200,000	500,000