Earnings per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Numerator:
Net income attributable to Biogen Idec Inc	$ 714,506	$ 490,670	$ 1,194,462	$ 917,419
Denominator:
Weighted average number of common shares outstanding	236,661,000	237,484,000	236,729,000	237,162,000
Effect of dilutive securities:
Dilutive potential common shares	700,000	1,200,000	900,000	1,400,000
Shares used in calculating diluted earnings per share	237,401,000	238,743,000	237,634,000	238,543,000
Stock options and employee stock purchase plan
Effect of dilutive securities:
Stock options and employee stock purchase plan	100,000	300,000	100,000	400,000
Time-vested restricted stock units
Effect of dilutive securities:
Stock options and employee stock purchase plan	400,000	600,000	500,000	700,000
Market stock units
Effect of dilutive securities:
Stock options and employee stock purchase plan	200,000	300,000	300,000	300,000