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Revenues by Product (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disaggregation of Revenue [Line Items]
Product Revenues	$ 3,526.3	$ 3,439.0	$ 3,356.5	$ 3,131.1	$ 3,307.0	$ 3,077.8	$ 3,078.4	$ 2,810.7	$ 13,452.9	$ 12,273.9	$ 11,448.8
TECFIDERA
Disaggregation of Revenue [Line Items]
Product Revenues									4,274.1	4,214.0	3,968.1
Interferon
Disaggregation of Revenue [Line Items]
Product Revenues									2,363.0	2,645.8	2,795.2
TYSABRI
Disaggregation of Revenue [Line Items]
Product Revenues									1,864.0	1,973.1	1,963.8
FAMPYRA
Disaggregation of Revenue [Line Items]
Product Revenues									92.7	91.6	84.9
ZINBRYTA
Disaggregation of Revenue [Line Items]
Product Revenues									1.4	52.7	7.8
MS Product Revenues
Disaggregation of Revenue [Line Items]
Product Revenues									8,595.2	8,977.2	8,819.8
SPINRAZA
Disaggregation of Revenue [Line Items]
Product Revenues									1,724.2	883.7	4.6
BENEPALI
Disaggregation of Revenue [Line Items]
Product Revenues									485.2	370.8	100.6
FLIXABI
Disaggregation of Revenue [Line Items]
Product Revenues									43.2	9.0	0.1
IMRALDI
Disaggregation of Revenue [Line Items]
Product Revenues									16.7	0.0	0.0
Biosimilars
Disaggregation of Revenue [Line Items]
Product Revenues									545.1	379.8	100.7
FUMADERM
Disaggregation of Revenue [Line Items]
Product Revenues									22.3	39.6	45.9
ELOCTATE
Disaggregation of Revenue [Line Items]
Product Revenues									0.0	48.4	513.2
ALPROLIX
Disaggregation of Revenue [Line Items]
Product Revenues									0.0	26.0	333.7
Hemophilia Products
Disaggregation of Revenue [Line Items]
Product Revenues									0.0	74.4	846.9
Product, net
Disaggregation of Revenue [Line Items]
Product Revenues	$ 2,825.7	$ 2,780.1	$ 2,757.5	$ 2,523.5	$ 2,712.4	$ 2,622.5	$ 2,639.7	$ 2,380.1	10,886.8	10,354.7	9,817.9
U.S | TECFIDERA
Disaggregation of Revenue [Line Items]
Product Revenues									3,253.2	3,294.0	3,169.4
U.S | Interferon
Disaggregation of Revenue [Line Items]
Product Revenues									1,668.3	1,889.1	1,980.3
U.S | TYSABRI
Disaggregation of Revenue [Line Items]
Product Revenues									1,025.0	1,113.8	1,182.9
U.S | FAMPYRA
Disaggregation of Revenue [Line Items]
Product Revenues									0.0	0.0	0.0
U.S | ZINBRYTA
Disaggregation of Revenue [Line Items]
Product Revenues									0.0	0.0	0.0
U.S | MS Product Revenues
Disaggregation of Revenue [Line Items]
Product Revenues									5,946.5	6,296.9	6,332.6
U.S | SPINRAZA
Disaggregation of Revenue [Line Items]
Product Revenues									854.0	657.0	4.6
U.S | BENEPALI
Disaggregation of Revenue [Line Items]
Product Revenues									0.0	0.0	0.0
U.S | FLIXABI
Disaggregation of Revenue [Line Items]
Product Revenues									0.0	0.0	0.0
U.S | IMRALDI
Disaggregation of Revenue [Line Items]
Product Revenues									0.0	0.0	0.0
U.S | Biosimilars
Disaggregation of Revenue [Line Items]
Product Revenues									0.0	0.0	0.0
U.S | FUMADERM
Disaggregation of Revenue [Line Items]
Product Revenues									0.0	0.0	0.0
U.S | ELOCTATE
Disaggregation of Revenue [Line Items]
Product Revenues									0.0	42.2	445.2
U.S | ALPROLIX
Disaggregation of Revenue [Line Items]
Product Revenues									0.0	21.0	268.0
U.S | Hemophilia Products
Disaggregation of Revenue [Line Items]
Product Revenues									0.0	63.2	713.2
U.S | Product, net
Disaggregation of Revenue [Line Items]
Product Revenues									6,800.5	7,017.1	7,050.4
Rest of world | TECFIDERA
Disaggregation of Revenue [Line Items]
Product Revenues									1,020.9	920.0	798.7
Rest of world | Interferon
Disaggregation of Revenue [Line Items]
Product Revenues									694.7	756.7	814.9
Rest of world | TYSABRI
Disaggregation of Revenue [Line Items]
Product Revenues									839.0	859.3	780.9
Rest of world | FAMPYRA
Disaggregation of Revenue [Line Items]
Product Revenues									92.7	91.6	84.9
Rest of world | ZINBRYTA
Disaggregation of Revenue [Line Items]
Product Revenues									1.4	52.7	7.8
Rest of world | MS Product Revenues
Disaggregation of Revenue [Line Items]
Product Revenues									2,648.7	2,680.3	2,487.2
Rest of world | SPINRAZA
Disaggregation of Revenue [Line Items]
Product Revenues									870.2	226.7	0.0
Rest of world | BENEPALI
Disaggregation of Revenue [Line Items]
Product Revenues									485.2	370.8	100.6
Rest of world | FLIXABI
Disaggregation of Revenue [Line Items]
Product Revenues									43.2	9.0	0.1
Rest of world | IMRALDI
Disaggregation of Revenue [Line Items]
Product Revenues									16.7	0.0	0.0
Rest of world | Biosimilars
Disaggregation of Revenue [Line Items]
Product Revenues									545.1	379.8	100.7
Rest of world | FUMADERM
Disaggregation of Revenue [Line Items]
Product Revenues									22.3	39.6	45.9
Rest of world | ELOCTATE
Disaggregation of Revenue [Line Items]
Product Revenues									0.0	6.2	68.0
Rest of world | ALPROLIX
Disaggregation of Revenue [Line Items]
Product Revenues									0.0	5.0	65.7
Rest of world | Hemophilia Products
Disaggregation of Revenue [Line Items]
Product Revenues									0.0	11.2	133.7
Rest of world | Product, net
Disaggregation of Revenue [Line Items]
Product Revenues									$ 4,086.3	$ 3,337.6	$ 2,767.5

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