Revenues by Product (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Disaggregation of Revenue [Line Items]
Product revenues	$ 3,616.7	$ 3,356.5	$ 7,106.5	$ 6,487.6
TECFIDERA
Disaggregation of Revenue [Line Items]
Product revenues	1,150.2	1,086.8	2,149.0	2,073.7
TECFIDERA | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	869.8	825.8	1,587.5	1,554.7
TECFIDERA | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	280.4	261.0	561.5	519.0
Interferon
Disaggregation of Revenue [Line Items]
Product revenues	554.4	625.5	1,055.3	1,175.8
Interferon | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	379.7	444.7	707.0	816.0
Interferon | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	174.7	180.8	348.3	359.8
TYSABRI
Disaggregation of Revenue [Line Items]
Product revenues	475.3	467.2	935.7	929.3
TYSABRI | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	264.3	265.5	509.3	515.2
TYSABRI | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	211.0	201.7	426.4	414.1
FAMPYRA
Disaggregation of Revenue [Line Items]
Product revenues	24.1	23.0	47.0	47.4
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	24.1	23.0	47.0	47.4
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	2,204.0	2,202.5	4,187.0	4,227.6
MS Product Revenues | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	1,513.8	1,536.0	2,803.8	2,885.9
MS Product Revenues | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	690.2	666.5	1,383.2	1,341.7
SPINRAZA
Disaggregation of Revenue [Line Items]
Product revenues	488.2	422.7	1,006.7	786.6
SPINRAZA | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	230.6	205.9	453.9	393.9
SPINRAZA | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	257.6	216.8	552.8	392.7
BENEPALI
Disaggregation of Revenue [Line Items]
Product revenues	120.3	115.6	244.3	236.5
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	120.3	115.6	244.3	236.5
IMRALDI
Disaggregation of Revenue [Line Items]
Product revenues	47.3	0.0	83.0	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	47.3	0.0	83.0	0.0
FLIXABI
Disaggregation of Revenue [Line Items]
Product revenues	16.8	11.2	31.5	17.8
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	16.8	11.2	31.5	17.8
Biosimilars
Disaggregation of Revenue [Line Items]
Product revenues	184.4	126.8	358.8	254.3
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	184.4	126.8	358.8	254.3
FUMADERM
Disaggregation of Revenue [Line Items]
Product revenues	3.7	5.5	7.8	12.5
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	3.7	5.5	7.8	12.5
Product, net
Disaggregation of Revenue [Line Items]
Product revenues	2,880.3	2,757.5	5,560.3	5,281.0
Product, net | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	1,744.4	1,741.9	3,257.7	3,279.8
Product, net | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	$ 1,135.9	$ 1,015.6	$ 2,302.6	$ 2,001.2