Revenues Revenues by Product (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Disaggregation of Revenue [Line Items]
Product, net	$ 2,523.5	$ 2,380.1
U.S.
Disaggregation of Revenue [Line Items]
Product, net	1,538.0	1,631.0
Rest of world
Disaggregation of Revenue [Line Items]
Product, net	985.5	749.1
TECFIDERA
Disaggregation of Revenue [Line Items]
Product, net	986.9	958.2
TECFIDERA | U.S.
Disaggregation of Revenue [Line Items]
Product, net	728.9	751.1
TECFIDERA | Rest of world
Disaggregation of Revenue [Line Items]
Product, net	258.0	207.1
Interferon
Disaggregation of Revenue [Line Items]
Product, net	550.3	648.3
Interferon | U.S.
Disaggregation of Revenue [Line Items]
Product, net	371.4	464.8
Interferon | Rest of world
Disaggregation of Revenue [Line Items]
Product, net	178.9	183.5
TYSABRI
Disaggregation of Revenue [Line Items]
Product, net	462.1	545.0
TYSABRI | U.S.
Disaggregation of Revenue [Line Items]
Product, net	249.7	305.5
TYSABRI | Rest of world
Disaggregation of Revenue [Line Items]
Product, net	212.4	239.5
FAMPYRA
Disaggregation of Revenue [Line Items]
Product, net	24.4	20.5
FAMPYRA | U.S.
Disaggregation of Revenue [Line Items]
Product, net	0.0	0.0
FAMPYRA | Rest of world
Disaggregation of Revenue [Line Items]
Product, net	24.4	20.5
ZINBRYTA
Disaggregation of Revenue [Line Items]
Product, net	1.4	10.7
ZINBRYTA | U.S.
Disaggregation of Revenue [Line Items]
Product, net	0.0	0.0
ZINBRYTA | Rest of world
Disaggregation of Revenue [Line Items]
Product, net	1.4	10.7
SPINRAZA
Disaggregation of Revenue [Line Items]
Product, net	363.9	47.4
SPINRAZA | U.S.
Disaggregation of Revenue [Line Items]
Product, net	188.0	46.4
SPINRAZA | Rest of world
Disaggregation of Revenue [Line Items]
Product, net	175.9	1.0
ELOCTATE
Disaggregation of Revenue [Line Items]
Product, net	0.0	48.4
ELOCTATE | U.S.
Disaggregation of Revenue [Line Items]
Product, net	0.0	42.2
ELOCTATE | Rest of world
Disaggregation of Revenue [Line Items]
Product, net	0.0	6.2
ALPROLIX
Disaggregation of Revenue [Line Items]
Product, net	0.0	26.0
ALPROLIX | U.S.
Disaggregation of Revenue [Line Items]
Product, net	0.0	21.0
ALPROLIX | Rest of world
Disaggregation of Revenue [Line Items]
Product, net	0.0	5.0
FUMADERM
Disaggregation of Revenue [Line Items]
Product, net	7.0	9.7
FUMADERM | U.S.
Disaggregation of Revenue [Line Items]
Product, net	0.0	0.0
FUMADERM | Rest of world
Disaggregation of Revenue [Line Items]
Product, net	7.0	9.7
BENEPALI
Disaggregation of Revenue [Line Items]
Product, net	120.9	65.3
BENEPALI | U.S.
Disaggregation of Revenue [Line Items]
Product, net	0.0	0.0
BENEPALI | Rest of world
Disaggregation of Revenue [Line Items]
Product, net	120.9	65.3
FLIXABI
Disaggregation of Revenue [Line Items]
Product, net	6.6	0.6
FLIXABI | U.S.
Disaggregation of Revenue [Line Items]
Product, net	0.0	0.0
FLIXABI | Rest of world
Disaggregation of Revenue [Line Items]
Product, net	$ 6.6	$ 0.6