Earnings per Share (Details) (USD $)	3 Months Ended													12 Months Ended
	Dec. 31, 2013		Sep. 30, 2013	Jun. 30, 2013		Mar. 31, 2013		Dec. 31, 2012		Sep. 30, 2012	Jun. 30, 2012		Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Numerator:
Net income attributable to Biogen Idec Inc	$ 457,300,000	[1],[2]	$ 487,600,000	$ 490,700,000	[3]	$ 426,700,000	[4]	$ 292,100,000	[5]	$ 398,400,000	$ 386,800,000	[6]	$ 302,700,000	$ 1,862,341,000	$ 1,380,033,000	$ 1,234,428,000
Adjustment for net income allocable to preferred stock														0	0	(500,000)
Net income used in calculating basic and diluted earnings per share														$ 1,862,300,000	$ 1,380,000,000	$ 1,233,900,000
Denominator:
Weighted average number of common shares outstanding														236,919,000	237,938,000	242,395,000
Effect of dilutive securities:
Dilutive potential common shares														1,400,000	1,800,000	2,600,000
Shares used in calculating diluted earnings per share														238,308,000	239,740,000	245,033,000
Earnings per share (Textual)
Repurchase of common stock														900,000	5,800,000	6,000,000
Stock options and employee stock purchase plan
Effect of dilutive securities:
Stock options and employee stock purchase plan														300,000	500,000	1,000,000
Time-vested restricted stock units
Effect of dilutive securities:
Stock options and employee stock purchase plan														800,000	1,000,000	1,300,000
Market stock units
Effect of dilutive securities:
Stock options and employee stock purchase plan														300,000	300,000	300,000

[1] Net income and net income attributable to Biogen Idec Inc. for the first quarter of 2012 includes a charge to research and development expense of $29.0 million related to an upfront payment made in connection with our development agreement entered into with Isis.

[2] 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.

[3] 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 TYSABRI rights from Elan on April 2, 2013 and net revenues related to sales of TECFIDERA, our new oral first-line treatment for people with relapsing forms of multiple sclerosis (MS), which was approved by the FDA in March 2013 and commenced commercial sales in April 2013.

[4] 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.

[5] Net income and net income attributable to Biogen Idec Inc. for the fourth quarter of 2012 includes the correction of an error that had accumulated over several prior years in our deferred tax accounting for capitalized interest which resulted in an expense of $29.0 million.

[6] Net income and net income attributable to Biogen Idec Inc. for the fourth quarter of 2012 includes a charge to research and development expense of $30.0 million related to an upfront payment made in connection with our development agreement entered into with Isis.