Earnings per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Numerator:
Net income attributable to Biogen Idec Inc	$ 490,670	$ 386,846	$ 917,419	$ 689,507
Denominator:
Weighted average number of common shares outstanding	237,484,000	238,988,000	237,162,000	239,389,000
Effect of dilutive securities:
Dilutive potential common shares	1,200,000	1,600,000	1,400,000	1,800,000
Shares used in calculating diluted earnings per share	238,743,000	240,622,000	238,543,000	241,245,000
Stock options and employee stock purchase plan [Member]
Effect of dilutive securities:
Stock options and employee stock purchase plan	300,000	600,000	400,000	600,000
Time-vested restricted stock units [Member]
Effect of dilutive securities:
Stock options and employee stock purchase plan	600,000	800,000	700,000	1,000,000
Market stock units [Member]
Effect of dilutive securities:
Stock options and employee stock purchase plan	300,000	200,000	300,000	200,000