Revenues Revenues by Product (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Disaggregation of Revenue [Line Items]
Product revenues	$ 3,356.5	$ 3,078.4	$ 6,487.6	$ 5,889.1
TECFIDERA
Disaggregation of Revenue [Line Items]
Product revenues	1,086.8	1,110.6	2,073.7	2,068.8
TECFIDERA | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	825.8	875.0	1,554.7	1,626.1
TECFIDERA | Rest of world
Disaggregation of Revenue [Line Items]
Product revenues	261.0	235.6	519.0	442.7
Interferon
Disaggregation of Revenue [Line Items]
Product revenues	625.5	690.6	1,175.8	1,338.9
Interferon | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	444.7	501.7	816.0	966.5
Interferon | Rest of world
Disaggregation of Revenue [Line Items]
Product revenues	180.8	188.9	359.8	372.4
TYSABRI
Disaggregation of Revenue [Line Items]
Product revenues	467.2	496.0	929.3	1,041.0
TYSABRI | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	265.5	289.4	515.2	594.9
TYSABRI | Rest of world
Disaggregation of Revenue [Line Items]
Product revenues	201.7	206.6	414.1	446.1
FAMPYRA
Disaggregation of Revenue [Line Items]
Product revenues	23.0	22.6	47.4	43.1
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	23.0	22.6	47.4	43.1
ZINBRYTA
Disaggregation of Revenue [Line Items]
Product revenues	0.0	16.1	1.4	26.8
ZINBRYTA | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	0.0	0.0	0.0	0.0
ZINBRYTA | Rest of world
Disaggregation of Revenue [Line Items]
Product revenues	0.0	16.1	1.4	26.8
SPINRAZA
Disaggregation of Revenue [Line Items]
Product revenues	422.7	202.9	786.6	250.3
SPINRAZA | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	205.9	194.8	393.9	241.2
SPINRAZA | Rest of world
Disaggregation of Revenue [Line Items]
Product revenues	216.8	8.1	392.7	9.1
ELOCTATE
Disaggregation of Revenue [Line Items]
Product revenues			0.0	48.4
ELOCTATE | U.S.
Disaggregation of Revenue [Line Items]
Product revenues			0.0	42.2
ELOCTATE | Rest of world
Disaggregation of Revenue [Line Items]
Product revenues			0.0	6.2
ALPROLIX
Disaggregation of Revenue [Line Items]
Product revenues			0.0	26.0
ALPROLIX | U.S.
Disaggregation of Revenue [Line Items]
Product revenues			0.0	21.0
ALPROLIX | Rest of world
Disaggregation of Revenue [Line Items]
Product revenues			0.0	5.0
FUMADERM
Disaggregation of Revenue [Line Items]
Product revenues	5.5	10.3	12.5	20.0
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	5.5	10.3	12.5	20.0
BENEPALI
Disaggregation of Revenue [Line Items]
Product revenues	115.6	88.7	236.5	154.0
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	115.6	88.7	236.5	154.0
FLIXABI
Disaggregation of Revenue [Line Items]
Product revenues	11.2	1.9	17.8	2.5
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	11.2	1.9	17.8	2.5
Product, net
Disaggregation of Revenue [Line Items]
Product revenues	2,757.5	2,639.7	5,281.0	5,019.8
Product, net | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	1,741.9	1,860.9	3,279.8	3,491.9
Product, net | Rest of world
Disaggregation of Revenue [Line Items]
Product revenues	$ 1,015.6	$ 778.8	$ 2,001.2	$ 1,527.9