Collaborative and Other Relationships - Collaborations (Details) $ in Millions	1 Months Ended				3 Months Ended							6 Months Ended
	Apr. 30, 2022 USD ($)	Aug. 31, 2021 USD ($)	Jun. 30, 2021 USD ($)	Dec. 31, 2019 product	Mar. 31, 2023 USD ($)	Jun. 30, 2022 USD ($)	Mar. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)	Sep. 30, 2021 USD ($)	Jun. 30, 2021 USD ($)	Mar. 31, 2020 USD ($)	Jun. 30, 2022 USD ($)	Jun. 30, 2021 USD ($)
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Research and Development Expense (Excluding Acquired in Process Cost)						$ 528.6				$ 585.1		$ 1,080.3	$ 1,099.3
Cost of sales, excluding amortization and impairment of acquired intangible assets						484.0				459.7		1,237.9	937.8
(Gain) loss on equity method investments												2.6	16.1
Samsung Bioepis
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
(Gain) loss on equity method investments						13.0				41.6		17.0	30.6
Loss recorded on Samsung Bioepis joint venture						5.9				34.3		2.6	16.1
Amortization						7.1				$ 7.3		14.4	14.5
ADUHELM
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Research and development expense												$ 275.0
Cost of sales, excluding amortization and impairment of acquired intangible assets						275.0
ADUHELM | Maximum | Forecast
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Research and development expense					$ 335.0
Eisai
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Percentage of future development costs related to Eisai										45.00%		45.00%
Collaboration agreement term							5 years					10 years
Cost of Goods Sold, Capacity Charges												$ 45.0
(Gain) loss on equity method investments						28.9				$ 40.1		210.6	40.1
Due from Related Parties						250.8		$ 285.4				250.8
Due to Related Parties						56.0		46.5				56.0
Eisai | BAN2401 and Elenbecestat
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Expense Incurred By Collaboration						78.6				62.2		155.6	117.7
Expense reflected within statements of income						39.3				31.1		77.8	58.8
Eisai | ADUHELM
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Expense reflected within statements of income						0.0				45.0		0.0	45.0
Additional milestone payment			$ 100.0			0.0				100.0		$ 0.0	100.0
(Gain) loss on equity method investments						160.0		59.0	$ 45.0	45.0
Samsung Bioepis
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Upfront and milestone payments made to collaborative partner											$ 100.0
Collaboration agreement term												5 years
Research and development expense											$ 63.0
Additional milestone payment	$ 50.0
Due from Related Parties						3.1		4.1				$ 3.1
Due to Related Parties						121.1		$ 148.7				$ 121.1
Contingent Commercialized Rights, Number Of Products | product				2
Sangamo Therapeutics, Inc. Agreement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Term of collaboration agreement												5 years
Other research and discovery
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Research and Development Expense (Excluding Acquired in Process Cost)						18.0				77.2		$ 37.5
InnoCare Pharma Limited (InnoCare) Agreement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Upfront and milestone payments made to collaborative partner		$ 125.0
Contingent additional milestone payment		$ 812.5
Research and development | Eisai | ADUHELM
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Expense incurred by the collaboration						37.2				42.1		81.4	89.1
Expense reflected within statements of income						20.5				23.2		44.8	49.0
Research and development | Denali Therapeutics
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Expense incurred by the collaboration						23.2				10.4		38.1	18.7
Expense reflected within statements of income						13.2				6.2		22.1	11.2
Research and development | Samsung Bioepis
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Expense incurred by the collaboration						2.6	$ 10.2			5.4		10.9
Expense reflected within statements of income						1.6	$ 7.6			3.4		7.1
Research and development | Sage Therapeutics Inc.
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Expense incurred by the collaboration						50.8				54.0		89.5	93.8
Expense reflected within statements of income						25.3				27.0		44.7	46.9
Research and development | UCB Pharma S.A. [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Expense incurred by the collaboration						15.6				16.1		33.2	33.0
Expense reflected within statements of income						7.8				8.1		16.6	16.5
Selling, general and administrative | Eisai | BAN2401 and Elenbecestat
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Expense incurred by the collaboration						23.4				4.3		39.3	10.0
Expense reflected within statements of income						11.7				2.1		19.7	5.0
Selling, general and administrative | Eisai | ADUHELM
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Expense incurred by the collaboration						40.8				125.6		135.8	237.4
Expense reflected within statements of income						21.1				67.6		72.0	127.9
Selling, general and administrative | Sage Therapeutics Inc.
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Expense incurred by the collaboration						23.0				10.5		41.4	15.8
Expense reflected within statements of income						$ 11.5				$ 5.2		$ 20.7	$ 7.9