Earnings per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Numerator:
Net income attributable to Biogen Idec Inc	$ 426,749	$ 302,661
Denominator:
Weighted average number of common shares outstanding	236,837,000	239,754,000
Effect of dilutive securities:
Dilutive potential common shares	1,500,000	2,000,000
Shares used in calculating diluted earnings per share	238,304,000	241,828,000
Stock options and employee stock purchase plan [Member]
Effect of dilutive securities:
Stock options and employee stock purchase plan	400,000	600,000
Time-vested restricted stock units [Member]
Effect of dilutive securities:
Stock options and employee stock purchase plan	800,000	1,100,000
Market stock units [Member]
Effect of dilutive securities:
Stock options and employee stock purchase plan	300,000	300,000