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Collaborative and Other Relationships - Samsung Bioepis (Details) $ in Millions, ₩ in Billions	1 Months Ended					3 Months Ended								12 Months Ended
	Jan. 31, 2020 USD ($)	Nov. 30, 2018 USD ($)	Nov. 30, 2018 KRW (₩)	Dec. 31, 2013 USD ($)	Feb. 29, 2012 USD ($)	Dec. 31, 2020 USD ($)	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2020 USD ($) product	Dec. 31, 2019 USD ($) product	Dec. 31, 2018 USD ($)	Dec. 31, 2020 KRW (₩)	Dec. 31, 2019 KRW (₩)	Jun. 30, 2018	Feb. 29, 2012 KRW (₩)
Collaborative arrangements and non-collaborative arrangement transactions
Payments to increase investment in Samsung Bioepis														$ 0.0	$ 0.0	$ 676.6
Eisai's share of aducanumab milestone expense reflected in collaboration profit sharing in our consolidated statements of income														$ 3.3	$ (77.4)	0.0
Number of ophthalmolgy biosimilar products \| product														2	2
Research and Development Expense (Excluding Acquired in Process Cost)														$ 3,990.9	$ 2,280.6	2,597.2
Collaboration profit (loss) sharing														232.9	241.6	185.0
Revenues						$ 2,852.6	$ 3,376.1	$ 3,681.6	$ 3,534.3	$ 3,671.3	$ 3,600.1	$ 3,616.7	$ 3,489.8	13,444.6	14,377.9	13,452.9
Samsung Bioepis
Collaborative arrangements and non-collaborative arrangement transactions
Upfront and milestone payments made to collaborative partner	$ 100.0
Research and development expense asset acquired	63.0
Prepaid research and discovery services	$ 37.0
Estimated additional payments upon achievement of development and commercial milestones														$ 195.0
Term of collaboration agreement				10 years										5 years
Option exercise fee														$ 60.0
Research and Development Expense (Excluding Acquired in Process Cost)				$ 46.0
Expected profit share percentage														50.00%
Collaboration profit (loss) sharing														$ 266.5	241.6	187.4
Revenues														20.9	106.2	$ 96.4
Due from related parties						5.1				85.0				5.1	85.0
Due to related parties						99.0				100.0				99.0	100.0
Samsung Bioepis \| Development Milestones
Collaborative arrangements and non-collaborative arrangement transactions
Total aducanumab collaboration third party milestone expense							$ 15.0
Samsung Bioepis
Collaborative arrangements and non-collaborative arrangement transactions
Contributions by third party					$ 250.0															₩ 280.5
Joint venture ownership percentage					85.00%
Investment in Samsung Bioepis					$ 45.0	$ 620.2				$ 580.2				$ 620.2	580.2		₩ 673.8	₩ 670.8		₩ 49.5
Samsung Bioepis, ownership percentage					15.00%	49.90%								49.90%			49.90%			15.00%
Samsung Bioepis, ownership percentage before additional purchase transaction																			5.00%
Payments to increase investment in Samsung Bioepis		$ 676.6	₩ 759.5
Total basis difference		$ 675.0
Net profits on investment														$ 5.3	79.4
Eisai's share of aducanumab milestone expense reflected in collaboration profit sharing in our consolidated statements of income														45.3	1.2
Amortization														$ 40.0	$ 78.2
Total aducanumab collaboration third party milestone expense				25.0
Net change in acquired and in-licensed rights and patents				$ 75.0
Samsung Bioepis \| Inventories
Collaborative arrangements and non-collaborative arrangement transactions
Equity method investment basis difference amortization period														1 year 6 months
Samsung Bioepis \| Completed technology
Collaborative arrangements and non-collaborative arrangement transactions
Equity method investment basis difference amortization period														15 years