Segment Information (Details) - USD ($) $ 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
Revenue by product
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
U.S
Revenue by product
Product, net									6,545.8	5,566.7	3,581.0
Rest of world
Revenue by product
Product, net									2,642.7	2,636.7	1,961.3
TECFIDERA
Revenue by product
Product, net									3,638.4	2,909.2	876.1
TECFIDERA | U.S
Revenue by product
Product, net									2,908.2	2,426.6	864.4
TECFIDERA | Rest of world
Revenue by product
Product, net									730.2	482.6	11.7
AVONEX
Revenue by product
Product, net									2,630.2	3,013.1	3,005.5
AVONEX | U.S
Revenue by product
Product, net									1,790.2	1,956.7	1,902.4
AVONEX | Rest of world
Revenue by product
Product, net									840.0	1,056.4	1,103.1
PLEGRIDY
Revenue by product
Product, net									338.5	44.5	0.0
PLEGRIDY | U.S
Revenue by product
Product, net									227.1	27.8	0.0
PLEGRIDY | Rest of world
Revenue by product
Product, net									111.4	16.7	0.0
TYSABRI
Revenue by product
Product, net									1,886.1	1,959.5	1,526.5
TYSABRI | U.S
Revenue by product
Product, net									1,103.1	1,025.1	814.2
TYSABRI | Rest of world
Revenue by product
Product, net									783.0	934.4	712.3
FAMPYRA
Revenue by product
Product, net									89.7	80.2	74.0
FAMPYRA | U.S
Revenue by product
Product, net									0.0	0.0	0.0
FAMPYRA | Rest of world
Revenue by product
Product, net									89.7	80.2	74.0
ELOCTATE
Revenue by product
Product, net									319.7	58.4	0.0
ELOCTATE | U.S
Revenue by product
Product, net									308.3	58.4	0.0
ELOCTATE | Rest of world
Revenue by product
Product, net									11.4	0.0	0.0
ALPROLIX
Revenue by product
Product, net									234.5	76.0	0.0
ALPROLIX | U.S
Revenue by product
Product, net									208.9	72.1	0.0
ALPROLIX | Rest of world
Revenue by product
Product, net									25.6	3.9	0.0
FUMADERM
Revenue by product
Product, net									51.4	62.5	60.2
FUMADERM | U.S
Revenue by product
Product, net									0.0	0.0	0.0
FUMADERM | Rest of world
Revenue by product
Product, net									$ 51.4	$ 62.5	$ 60.2