Accumulated Other Comprehensive Income (Loss) Accumulated Other Comprehensive Income (Loss) (Details 1) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other income (expense)	$ 4,861	$ (10,428)	$ (740)	$ (24,885)
Income tax benefit (expense)	268,521	159,140	446,935	224,648
Product, net	2,056,292	1,385,918	3,799,057	2,481,697
Net income attributable to Biogen Idec Inc.	714,506	490,670	1,194,462	917,419
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net income attributable to Biogen Idec Inc.	700	(100)	(3,500)	3,700
Accumulated Net Unrealized Investment Gain (Loss) | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other income (expense)	10,100	0	10,200	4,300
Income tax benefit (expense)	(3,600)	0	(3,600)	(1,500)
Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income tax benefit (expense)	(500)	(200)	(100)	(300)
Product, net	$ (5,300)	$ 100	$ (10,000)	$ 1,200