Segment Information (Details) - USD ($) $ 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
Revenue by product
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
U.S
Revenue by product
Product, net									7,050.4	6,545.8	5,566.7
Rest of world
Revenue by product
Product, net									2,767.5	2,642.7	2,636.7
TECFIDERA
Revenue by product
Product, net									3,968.1	3,638.4	2,909.2
TECFIDERA | U.S
Revenue by product
Product, net									3,169.4	2,908.2	2,426.6
TECFIDERA | Rest of world
Revenue by product
Product, net									798.7	730.2	482.6
AVONEX
Revenue by product
Product, net									2,313.5	2,630.2	3,013.1
AVONEX | U.S
Revenue by product
Product, net									1,675.3	1,790.2	1,956.7
AVONEX | Rest of world
Revenue by product
Product, net									638.2	840.0	1,056.4
PLEGRIDY
Revenue by product
Product, net									481.7	338.5	44.5
PLEGRIDY | U.S
Revenue by product
Product, net									305.0	227.1	27.8
PLEGRIDY | Rest of world
Revenue by product
Product, net									176.7	111.4	16.7
TYSABRI
Revenue by product
Product, net									1,963.8	1,886.1	1,959.5
TYSABRI | U.S
Revenue by product
Product, net									1,182.9	1,103.1	1,025.1
TYSABRI | Rest of world
Revenue by product
Product, net									780.9	783.0	934.4
FAMPYRA
Revenue by product
Product, net									84.9	89.7	80.2
FAMPYRA | U.S
Revenue by product
Product, net									0.0	0.0	0.0
FAMPYRA | Rest of world
Revenue by product
Product, net									84.9	89.7	80.2
ZINBRYTA
Revenue by product
Product, net									7.8	0.0	0.0
ZINBRYTA | U.S
Revenue by product
Product, net									0.0	0.0	0.0
ZINBRYTA | Rest of world
Revenue by product
Product, net									7.8	0.0	0.0
ELOCTATE
Revenue by product
Product, net									513.2	319.7	58.4
ELOCTATE | U.S
Revenue by product
Product, net									445.2	308.3	58.4
ELOCTATE | Rest of world
Revenue by product
Product, net									68.0	11.4	0.0
ALPROLIX
Revenue by product
Product, net									333.7	234.5	76.0
ALPROLIX | U.S
Revenue by product
Product, net									268.0	208.9	72.1
ALPROLIX | Rest of world
Revenue by product
Product, net									65.7	25.6	3.9
FUMADERM
Revenue by product
Product, net									45.9	51.4	62.5
FUMADERM | U.S
Revenue by product
Product, net									0.0	0.0	0.0
FUMADERM | Rest of world
Revenue by product
Product, net									45.9	51.4	62.5
SPINRAZA
Revenue by product
Product, net									4.6	0.0	0.0
SPINRAZA | U.S
Revenue by product
Product, net									4.6	0.0	0.0
SPINRAZA | Rest of world
Revenue by product
Product, net									0.0	0.0	0.0
BENEPALI
Revenue by product
Product, net									100.6	0.0	0.0
BENEPALI | U.S
Revenue by product
Product, net									0.0	0.0	0.0
BENEPALI | Rest of world
Revenue by product
Product, net									100.6	0.0	0.0
FLIXABI
Revenue by product
Product, net									0.1	0.0	0.0
FLIXABI | U.S
Revenue by product
Product, net									0.0	0.0	0.0
FLIXABI | Rest of world
Revenue by product
Product, net									$ 0.1	$ 0.0	$ 0.0