Revenues by Product (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Disaggregation of Revenue [Line Items]
Product Revenues	$ 3,671.3	$ 3,600.1	$ 3,616.7	$ 3,489.8	$ 3,526.3	$ 3,439.0	$ 3,356.5	$ 3,131.1	$ 14,377.9	$ 13,452.9	$ 12,273.9
TECFIDERA
Disaggregation of Revenue [Line Items]
Product Revenues									4,432.7	4,274.1	4,214.0
Interferon
Disaggregation of Revenue [Line Items]
Product Revenues									2,101.8	2,363.0	2,645.8
TYSABRI
Disaggregation of Revenue [Line Items]
Product Revenues									1,892.2	1,864.0	1,973.1
VUMERITY
Disaggregation of Revenue [Line Items]
Product Revenues									5.5	0.0	0.0
FAMPYRA
Disaggregation of Revenue [Line Items]
Product Revenues									97.1	92.7	91.6
ZINBRYTA
Disaggregation of Revenue [Line Items]
Product Revenues									0.0	1.4	52.7
MS Product Revenues
Disaggregation of Revenue [Line Items]
Product Revenues									8,529.3	8,595.2	8,977.2
SPINRAZA
Disaggregation of Revenue [Line Items]
Product Revenues									2,097.0	1,724.2	883.7
BENEPALI
Disaggregation of Revenue [Line Items]
Product Revenues									486.2	485.2	370.8
FLIXABI
Disaggregation of Revenue [Line Items]
Product Revenues									68.1	43.2	9.0
IMRALDI
Disaggregation of Revenue [Line Items]
Product Revenues									184.0	16.7	0.0
Biosimilars
Disaggregation of Revenue [Line Items]
Product Revenues									738.3	545.1	379.8
FUMADERM
Disaggregation of Revenue [Line Items]
Product Revenues									15.2	22.3	39.6
ELOCTATE
Disaggregation of Revenue [Line Items]
Product Revenues									0.0	0.0	48.4
ALPROLIX
Disaggregation of Revenue [Line Items]
Product Revenues									0.0	0.0	26.0
Hemophilia Products
Disaggregation of Revenue [Line Items]
Product Revenues									0.0	0.0	74.4
Product, net
Disaggregation of Revenue [Line Items]
Product Revenues	$ 2,924.8	$ 2,894.7	$ 2,880.3	$ 2,680.0	$ 2,825.7	$ 2,780.1	$ 2,757.5	$ 2,523.5	11,379.8	10,886.8	10,354.7
U.S | TECFIDERA
Disaggregation of Revenue [Line Items]
Product Revenues									3,306.5	3,253.2	3,294.0
U.S | Interferon
Disaggregation of Revenue [Line Items]
Product Revenues									1,426.6	1,668.3	1,889.1
U.S | TYSABRI
Disaggregation of Revenue [Line Items]
Product Revenues									1,041.8	1,025.0	1,113.8
U.S | VUMERITY
Disaggregation of Revenue [Line Items]
Product Revenues									5.5	0.0	0.0
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									5,780.4	5,946.5	6,296.9
U.S | SPINRAZA
Disaggregation of Revenue [Line Items]
Product Revenues									933.4	854.0	657.0
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	0.0	42.2
U.S | ALPROLIX
Disaggregation of Revenue [Line Items]
Product Revenues									0.0	0.0	21.0
U.S | Hemophilia Products
Disaggregation of Revenue [Line Items]
Product Revenues									0.0	0.0	63.2
U.S | Product, net
Disaggregation of Revenue [Line Items]
Product Revenues									6,713.8	6,800.5	7,017.1
Rest of world | TECFIDERA
Disaggregation of Revenue [Line Items]
Product Revenues									1,126.2	1,020.9	920.0
Rest of world | Interferon
Disaggregation of Revenue [Line Items]
Product Revenues									675.2	694.7	756.7
Rest of world | TYSABRI
Disaggregation of Revenue [Line Items]
Product Revenues									850.4	839.0	859.3
Rest of world | VUMERITY
Disaggregation of Revenue [Line Items]
Product Revenues									0.0	0.0	0.0
Rest of world | FAMPYRA
Disaggregation of Revenue [Line Items]
Product Revenues									97.1	92.7	91.6
Rest of world | ZINBRYTA
Disaggregation of Revenue [Line Items]
Product Revenues									0.0	1.4	52.7
Rest of world | MS Product Revenues
Disaggregation of Revenue [Line Items]
Product Revenues									2,748.9	2,648.7	2,680.3
Rest of world | SPINRAZA
Disaggregation of Revenue [Line Items]
Product Revenues									1,163.6	870.2	226.7
Rest of world | BENEPALI
Disaggregation of Revenue [Line Items]
Product Revenues									486.2	485.2	370.8
Rest of world | FLIXABI
Disaggregation of Revenue [Line Items]
Product Revenues									68.1	43.2	9.0
Rest of world | IMRALDI
Disaggregation of Revenue [Line Items]
Product Revenues									184.0	16.7	0.0
Rest of world | Biosimilars
Disaggregation of Revenue [Line Items]
Product Revenues									738.3	545.1	379.8
Rest of world | FUMADERM
Disaggregation of Revenue [Line Items]
Product Revenues									15.2	22.3	39.6
Rest of world | ELOCTATE
Disaggregation of Revenue [Line Items]
Product Revenues									0.0	0.0	6.2
Rest of world | ALPROLIX
Disaggregation of Revenue [Line Items]
Product Revenues									0.0	0.0	5.0
Rest of world | Hemophilia Products
Disaggregation of Revenue [Line Items]
Product Revenues									0.0	0.0	11.2
Rest of world | Product, net
Disaggregation of Revenue [Line Items]
Product Revenues									$ 4,666.0	$ 4,086.3	$ 3,337.6