Revenues Revenues by Product (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Disaggregation of Revenue [Line Items]
Product revenues	$ 3,439.0	$ 3,077.8	$ 9,926.6	$ 8,966.9
TECFIDERA
Disaggregation of Revenue [Line Items]
Product revenues	1,090.0	1,069.6	3,163.7	3,138.4
TECFIDERA | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	842.1	836.3	2,396.8	2,462.4
TECFIDERA | Rest of world
Disaggregation of Revenue [Line Items]
Product revenues	247.9	233.3	766.9	676.0
Interferon
Disaggregation of Revenue [Line Items]
Product revenues	590.1	662.0	1,765.9	2,000.9
Interferon | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	421.5	473.3	1,237.5	1,439.8
Interferon | Rest of world
Disaggregation of Revenue [Line Items]
Product revenues	168.6	188.7	528.4	561.1
TYSABRI
Disaggregation of Revenue [Line Items]
Product revenues	470.2	469.4	1,399.5	1,510.4
TYSABRI | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	253.0	266.8	768.2	861.7
TYSABRI | Rest of world
Disaggregation of Revenue [Line Items]
Product revenues	217.2	202.6	631.3	648.7
FAMPYRA
Disaggregation of Revenue [Line Items]
Product revenues	22.5	24.3	69.9	67.4
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	22.5	24.3	69.9	67.4
ZINBRYTA
Disaggregation of Revenue [Line Items]
Product revenues	0.0	14.2	1.4	41.0
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	14.2	1.4	41.0
SPINRAZA
Disaggregation of Revenue [Line Items]
Product revenues	467.7	270.9	1,254.3	521.2
SPINRAZA | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	223.9	197.6	617.8	438.8
SPINRAZA | Rest of world
Disaggregation of Revenue [Line Items]
Product revenues	243.8	73.3	636.5	82.4
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	4.8	10.7	17.3	30.7
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	4.8	10.7	17.3	30.7
BENEPALI
Disaggregation of Revenue [Line Items]
Product revenues	123.4	99.2	359.9	253.2
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	123.4	99.2	359.9	253.2
FLIXABI
Disaggregation of Revenue [Line Items]
Product revenues	11.4	2.2	29.2	4.7
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	11.4	2.2	29.2	4.7
Product, net
Disaggregation of Revenue [Line Items]
Product revenues	2,780.1	2,622.5	8,061.1	7,642.3
Product, net | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	1,740.5	1,774.0	5,020.3	5,265.9
Product, net | Rest of world
Disaggregation of Revenue [Line Items]
Product revenues	$ 1,039.6	$ 848.5	$ 3,040.8	$ 2,376.4