Earnings per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended											12 Months Ended
	Dec. 31, 2014		Sep. 30, 2014	Jun. 30, 2014		Mar. 31, 2014		Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Numerator:
Net income attributable to Biogen Idec Inc	$ 883,500	[1]	$ 856,900	$ 714,500	[2]	$ 480,000	[3]	$ 457,300	$ 487,600	$ 490,700	$ 426,700	$ 2,934,784	$ 1,862,341	$ 1,380,033
Denominator:
Weighted average number of common shares outstanding												236,359,000	236,919,000	237,938,000
Effect of dilutive securities:
Dilutive potential common shares												800,000	1,400,000	1,800,000
Shares used in calculating diluted earnings per share												237,176,000	238,308,000	239,740,000
Earnings per share (Textual)
Repurchase of common stock												1,000,000	900,000	5,800,000
Stock options and employee stock purchase plan
Effect of dilutive securities:
Stock options and employee stock purchase plan												100,000	300,000	500,000
Time-vested restricted stock units
Effect of dilutive securities:
Stock options and employee stock purchase plan												500,000	800,000	1,000,000
Market stock units
Effect of dilutive securities:
Stock options and employee stock purchase plan												200,000	300,000	300,000

[1] Net income and net income attributable to Biogen Idec Inc., for the third quarter of 2013, includes a charge to research and development expense of $75.0 million related to an upfront payment made in connection with our collaboration agreement entered into with Isis.

[2] Product revenues and total revenues for the second, third and fourth quarters of 2013 includes 100% of net revenues related to sales of TYSABRI as a result of our acquisition of all remaining rights to TYSABRI from Elan on April 2, 2013 and net revenues related to sales of TECFIDERA, which was approved by the FDA in March 2013 and commenced commercial sales in April 2013.

[3] Our share of revenues from unconsolidated joint business reflects a charge of $41.5 million for damages and interest awarded to Hoechst in Genentech's arbitration with Hoechst for RITUXAN.