Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Selected Quarterly Financial Information [Abstract]
Product Revenues		$ 3,671.3	$ 3,600.1	$ 3,616.7	$ 3,489.8	$ 3,526.3	$ 3,439.0	$ 3,356.5	$ 3,131.1	$ 14,377.9	$ 13,452.9	$ 12,273.9
Gross Profit	[1]	3,224.2	3,170.1	3,140.4	2,887.8	3,037.8	2,978.2	2,935.5	2,685.1	12,422.5	11,636.6
Net income		1,439.7	1,545.9	1,494.1	1,408.8	944.9	1,442.9	915.0	1,171.2	5,888.5	4,474.0	2,670.1
Net income attributable to Biogen Idec Inc		$ 1,439.7	$ 1,545.9	$ 1,494.1	$ 1,408.8	$ 946.8	$ 1,444.4	$ 866.6	$ 1,172.9	$ 5,888.5	$ 4,430.7	$ 2,539.1
Basic earnings per share attributable to Biogen Inc.		$ 8.10	$ 8.40	$ 7.85	$ 7.17	$ 4.74	$ 7.17	$ 4.18	$ 5.55	$ 31.47	$ 21.63	$ 11.94
Diluted earnings per share attributable to Biogen Inc.		$ 8.08	$ 8.39	$ 7.85	$ 7.15	$ 4.73	$ 7.15	$ 4.18	$ 5.54	$ 31.42	$ 21.58	$ 11.92
Basic earnings per share attributable to Biogen Inc.		177.8	184.0	190.3	196.6	199.8	201.4	207.1	211.4	187.1	204.9	212.6
Diluted earnings per share attributable to Biogen Inc.		178.2	184.2	190.4	197.0	200.3	201.9	207.3	211.7	187.4	205.3	213.0
Revenues from anti-cd20 therapeutic programs
Selected Quarterly Financial Information [Abstract]
Product Revenues		$ 600.8	$ 595.8	$ 576.4	$ 517.4	$ 534.9	$ 511.7	$ 490.4	$ 443.2	$ 2,290.4	$ 1,980.2	$ 1,559.2
Other revenue
Selected Quarterly Financial Information [Abstract]
Product Revenues		145.7	109.6	160.0	292.4	165.7	147.2	108.6	164.4	707.7	585.9	360.0
Product, net
Selected Quarterly Financial Information [Abstract]
Product Revenues		$ 2,924.8	$ 2,894.7	$ 2,880.3	$ 2,680.0	$ 2,825.7	$ 2,780.1	$ 2,757.5	$ 2,523.5	$ 11,379.8	$ 10,886.8	$ 10,354.7

[1] (1) Gross profit is calculated as total revenues less cost of sales, excluding amortization and impairment of acquired intangible assets