Revenues by Product (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Disaggregation of Revenue [Line Items]
Product revenues	$ 3,600.1	$ 3,439.0	$ 10,706.6	$ 9,926.6
TECFIDERA
Disaggregation of Revenue [Line Items]
Product revenues	1,122.4	1,090.0	3,271.4	3,163.7
TECFIDERA | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	842.0	842.1	2,429.5	2,396.8
TECFIDERA | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	280.4	247.9	841.9	766.9
Interferon
Disaggregation of Revenue [Line Items]
Product revenues	530.0	590.1	1,585.3	1,765.9
Interferon | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	360.3	421.5	1,067.3	1,237.5
Interferon | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	169.7	168.6	518.0	528.4
TYSABRI
Disaggregation of Revenue [Line Items]
Product revenues	483.6	470.2	1,419.3	1,399.5
TYSABRI | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	263.0	253.0	772.3	768.2
TYSABRI | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	220.6	217.2	647.0	631.3
FAMPYRA
Disaggregation of Revenue [Line Items]
Product revenues	24.2	22.5	71.2	69.9
FAMPYRA | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	0.0	0.0	0.0	0.0
FAMPYRA | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	24.2	22.5	71.2	69.9
ZINBRYTA
Disaggregation of Revenue [Line Items]
Product revenues			0.0	1.4
ZINBRYTA | U.S.
Disaggregation of Revenue [Line Items]
Product revenues			0.0	0.0
ZINBRYTA | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues			0.0	1.4
MS Product Revenues
Disaggregation of Revenue [Line Items]
Product revenues	2,160.2	2,172.8	6,347.2	6,400.4
MS Product Revenues | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	1,465.3	1,516.6	4,269.1	4,402.5
MS Product Revenues | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	694.9	656.2	2,078.1	1,997.9
SPINRAZA
Disaggregation of Revenue [Line Items]
Product revenues	547.1	467.7	1,553.8	1,254.3
SPINRAZA | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	236.7	223.9	690.6	617.8
SPINRAZA | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	310.4	243.8	863.2	636.5
BENEPALI
Disaggregation of Revenue [Line Items]
Product revenues	115.9	123.4	360.2	359.9
BENEPALI | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	0.0	0.0	0.0	0.0
BENEPALI | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	115.9	123.4	360.2	359.9
IMRALDI
Disaggregation of Revenue [Line Items]
Product revenues	49.3	0.0	132.3	0.0
IMRALDI | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	0.0	0.0	0.0	0.0
IMRALDI | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	49.3	0.0	132.3	0.0
FLIXABI
Disaggregation of Revenue [Line Items]
Product revenues	18.4	11.4	49.9	29.2
FLIXABI | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	0.0	0.0	0.0	0.0
FLIXABI | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	18.4	11.4	49.9	29.2
Biosimilars
Disaggregation of Revenue [Line Items]
Product revenues	183.6	134.8	542.4	389.1
Biosimilars | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	0.0	0.0	0.0	0.0
Biosimilars | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	183.6	134.8	542.4	389.1
FUMADERM
Disaggregation of Revenue [Line Items]
Product revenues	3.8	4.8	11.6	17.3
FUMADERM | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	0.0	0.0	0.0	0.0
FUMADERM | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	3.8	4.8	11.6	17.3
Product, net
Disaggregation of Revenue [Line Items]
Product revenues	2,894.7	2,780.1	8,455.0	8,061.1
Product, net | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	1,702.0	1,740.5	4,959.7	5,020.3
Product, net | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	$ 1,192.7	$ 1,039.6	$ 3,495.3	$ 3,040.8