Earnings per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Numerator:
Net income attributable to Biogen Idec Inc	$ 479,956	$ 426,749
Denominator:
Weighted average number of common shares outstanding	236,786,000	236,837,000
Effect of dilutive securities:
Dilutive potential common shares	1,000,000	1,500,000
Shares used in calculating diluted earnings per share	237,849,000	238,304,000
Stock options and employee stock purchase plan
Effect of dilutive securities:
Stock options and employee stock purchase plan	100,000	400,000
Time-vested restricted stock units
Effect of dilutive securities:
Stock options and employee stock purchase plan	600,000	800,000
Market stock units
Effect of dilutive securities:
Stock options and employee stock purchase plan	300,000	300,000