Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Selected Quarterly Financial Information [Abstract]
Product, net		$ 2,502.9	$ 2,539.6	$ 2,466.0	$ 2,309.4	$ 2,425.9	$ 2,391.7	$ 2,198.6	$ 2,172.3	$ 9,817.9	$ 9,188.5	$ 8,203.4
Revenues from anti-CD20 therapeutic programs		318.2	317.6	349.2	329.5	333.9	337.2	337.5	330.6	1,314.5	1,339.2	1,195.4
Other		50.9	98.6	79.0	87.9	79.5	49.0	55.6	52.0	316.4	236.1	304.5
Total revenues		2,872.0	2,955.8	2,894.2	2,726.8	2,839.3	2,777.9	2,591.6	2,555.0	11,448.8	10,763.8	9,703.3
Gross Profit	[1]	2,493.5	2,538.9	2,523.9	2,413.8	2,507.5	2,467.9	2,305.5	2,242.6	9,970.1	9,523.4
Net income		647.9	1,030.2	1,048.4	969.2	828.7	1,019.5	924.8	820.2	3,695.7	3,593.2	2,941.6
Net income attributable to Biogen Idec Inc		$ 649.2	$ 1,032.9	$ 1,049.8	$ 970.9	$ 831.6	$ 965.6	$ 927.3	$ 822.5	$ 3,702.8	$ 3,547.0	$ 2,934.8
Basic earnings per share attributable to Biogen Inc.		$ 3.00	$ 4.72	$ 4.79	$ 4.44	$ 3.77	$ 4.16	$ 3.94	$ 3.50	$ 16.96	$ 15.38	$ 12.42
Diluted earnings per share attributable to Biogen Inc.		$ 2.99	$ 4.71	$ 4.79	$ 4.43	$ 3.77	$ 4.15	$ 3.93	$ 3.49	$ 16.93	$ 15.34	$ 12.37
Basic earnings per share attributable to Biogen Inc.		216.6	218.9	219.1	218.9	220.4	232.2	235.3	235.0	218.4	230.7	236.4
Diluted earnings per share attributable to Biogen Inc.		217.0	219.4	219.4	219.3	220.8	232.6	235.7	235.6	218.8	231.2	237.2

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