Segment Information (Details) (USD $) In Thousands, 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
Revenue by product
Product, net	$ 2,287,000	[1]	$ 2,117,300	$ 2,056,300	[2]	$ 1,742,800	[3]	$ 1,607,100	$ 1,453,600	$ 1,385,900	$ 1,095,800	$ 8,203,404	$ 5,542,331	$ 4,166,074
U.S
Revenue by product
Product, net												5,566,700	3,581,000	2,176,800
Rest of world
Revenue by product
Product, net												2,636,700	1,961,300	1,989,300
AVONEX
Revenue by product
Product, net												3,013,100	3,005,500	2,913,100
AVONEX | U.S
Revenue by product
Product, net												1,956,700	1,902,400	1,793,700
AVONEX | Rest of world
Revenue by product
Product, net												1,056,400	1,103,100	1,119,400
PLEGRIDY
Revenue by product
Product, net												44,500	0	0
PLEGRIDY | U.S
Revenue by product
Product, net												27,800	0	0
PLEGRIDY | Rest of world
Revenue by product
Product, net												16,700	0	0
TECFIDERA
Revenue by product
Product, net												2,909,200	876,100	0
TECFIDERA | U.S
Revenue by product
Product, net												2,426,600	864,400	0
TECFIDERA | Rest of world
Revenue by product
Product, net												482,600	11,700	0
TYSABRI
Revenue by product
Product, net												1,959,500	1,526,500	1,135,900
TYSABRI | U.S
Revenue by product
Product, net												1,025,100	814,200	383,100
TYSABRI | Rest of world
Revenue by product
Product, net												934,400	712,300	752,800
FAMPYRA
Revenue by product
Product, net												80,200	74,000	57,400
FAMPYRA | U.S
Revenue by product
Product, net												0	0	0
FAMPYRA | Rest of world
Revenue by product
Product, net												80,200	74,000	57,400
ALPROLIX
Revenue by product
Product, net												76,000	0	0
ALPROLIX | U.S
Revenue by product
Product, net												72,100	0	0
ALPROLIX | Rest of world
Revenue by product
Product, net												3,900	0	0
ELOCTATE
Revenue by product
Product, net												58,400	0	0
ELOCTATE | U.S
Revenue by product
Product, net												58,400	0	0
ELOCTATE | Rest of world
Revenue by product
Product, net												0	0	0
FUMADERM
Revenue by product
Product, net												62,500	60,200	59,700
FUMADERM | U.S
Revenue by product
Product, net												0	0	0
FUMADERM | Rest of world
Revenue by product
Product, net												$ 62,500	$ 60,200	$ 59,700

[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.