Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Selected Quarterly Financial Information [Abstract]
Product, net	$ 2,425.9	$ 2,391.7	$ 2,198.6	$ 2,172.3	$ 2,287.0	$ 2,117.3	$ 2,056.3	$ 1,742.8	$ 9,188.5	$ 8,203.4	$ 5,542.3
Unconsolidated joint business revenues	333.9	337.2	337.5	330.6	304.5	290.7	303.3	296.9	1,339.2	1,195.4	1,126.0
Other	79.5	49.0	55.6	52.0	49.2	103.4	61.9	90.1	236.1	304.5	263.9
Total revenues	2,839.3	2,777.9	2,591.6	2,555.0	2,640.7	2,511.4	2,421.5	2,129.8	10,763.8	9,703.3	6,932.2
Gross Profit	2,507.5	2,467.9	2,305.5	2,242.6	2,343.4	2,208.8	2,129.6	1,850.5	9,523.4	8,532.3
Net income	828.7	1,019.5	924.8	820.2	882.6	856.1	723.1	479.7	3,593.2	2,941.6	1,862.3
Net income attributable to Biogen Idec Inc	$ 831.6	$ 965.6	$ 927.3	$ 822.5	$ 883.5	$ 856.9	$ 714.5	$ 480.0	$ 3,547.0	$ 2,934.8	$ 1,862.3
Basic earnings per share attributable to Biogen Inc.	$ 3.77	$ 4.16	$ 3.94	$ 3.50	$ 3.75	$ 3.63	$ 3.02	$ 2.03	$ 15.38	$ 12.42	$ 7.86
Diluted earnings per share attributable to Biogen Inc.	$ 3.77	$ 4.15	$ 3.93	$ 3.49	$ 3.74	$ 3.62	$ 3.01	$ 2.02	$ 15.34	$ 12.37	$ 7.81
Basic earnings per share attributable to Biogen Inc.	220.4	232.2	235.3	235.0	235.5	236.2	236.7	236.8	230.7	236.4	236.9
Diluted earnings per share attributable to Biogen Inc.	220.8	232.6	235.7	235.6	236.3	237.0	237.4	237.8	231.2	237.2	238.3