Collaborative and Other Relationships - Collaborations (Details) $ / shares in Units, shares in Millions, $ in Millions	1 Months Ended			3 Months Ended			6 Months Ended
	Jun. 30, 2021 USD ($)	Nov. 30, 2020 USD ($) $ / shares shares	Feb. 29, 2020 USD ($) $ / shares shares	Jun. 30, 2021 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Jun. 30, 2021 USD ($) product $ / shares	Jun. 30, 2020 USD ($)	Jan. 31, 2020 USD ($)
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Research and Development Expense (Excluding Acquired in Process Cost)				$ 585.1	$ 647.6		$ 1,099.3	$ 1,123.9
Eisai
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Number of product candidates | product							2
Percentage of future development costs related to Eisai				45.00%	45.00%
Loss on research and development contracts terminated with Eisai						$ 45.0
Additional milestone payment					$ 75.0
Eisai | BAN2401 and Elenbecestat
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Expense Incurred By Collaboration				$ 62.2	33.9		$ 117.7	77.5
Expense reflected within statements of income				31.1	17.0		58.8	38.7
Eisai | ADUHELM
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Expense reflected within statements of income				45.0	33.8		45.0	33.8
Additional milestone payment	$ 100.0			100.0	75.0		100.0	75.0
Denali Therapeutics
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Global licensing collaboration agreement, amount, shares purchased (in shares)							$ 465.0
Global licensing collaboration agreement, shares purchased per share (in usd per share) | $ / shares							$ 34.94
Upfront and milestone payments made to collaborative partner							$ 560.0
Global Licensing Collaboration Agreement, Research, Development, Regulatory and Commercial Milestone Payments							1,100.0
Research and development expense							$ 41.3
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
Prepaid research and development expenses									$ 37.0
Additional milestone payment				15.0
Sage Therapeutics Inc.
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Global licensing collaboration agreement, amount, shares purchased (in shares)		$ 650.0
Global licensing collaboration agreement, shares, purchased (in shares) | shares		6.2
Upfront and milestone payments made to collaborative partner		$ 875.0
Global Licensing Collaboration Agreement, Research, Development, Regulatory and Commercial Milestone Payments		1,600.0
Research and development expense		$ 209.0
Global licensing collaboration agreement, purchase price per share (in usd per share) | $ / shares		$ 104.14
Sangamo Therapeutics, Inc. Agreement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Global licensing collaboration agreement, amount, shares purchased (in shares)			$ 225.0
Global licensing collaboration agreement, shares, purchased (in shares) | shares			24.0
Global licensing collaboration agreement, shares purchased per share (in usd per share) | $ / shares			$ 9.21
Upfront and milestone payments made to collaborative partner			$ 125.0
Global Licensing Collaboration Agreement, Research, Development, Regulatory and Commercial Milestone Payments			2,400.0
Global Licensing Collaboration Agreement, Payment, Selection of Targets			80.0
Global Licensing Collaboration Agreement, Payment, Achievement of First Commercial Sale Milestones			1,900.0
Global Licensing Collaboration Agreement, Payment, Achievement of Specific Sales Based Milestones			380.0
Research and development expense			$ 83.0
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)				77.2	9.6		$ 77.2	9.6
Research and development | Eisai | ADUHELM
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Expense incurred by the collaboration				42.1	35.9		89.1	55.0
Expense reflected within statements of income				23.2	19.7		49.0	30.3
Research and development | Denali Therapeutics
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Expense incurred by the collaboration				10.4	0.0		18.7	0.0
Expense reflected within statements of income				6.2	0.0		11.2	0.0
Research and development | Sage Therapeutics Inc.
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Expense incurred by the collaboration				54.0	0.0		93.8	0.0
Expense reflected within statements of income				27.0	0.0		46.9	0.0
Research and development | UCB Pharma S.A. [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Expense incurred by the collaboration				16.1	6.5		33.0	25.5
Expense reflected within statements of income				8.1	3.2		16.5	12.7
Selling, general and administrative | Eisai | BAN2401 and Elenbecestat
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Expense incurred by the collaboration				4.3	1.4		10.0	6.3
Expense reflected within statements of income				2.1	0.7		5.0	3.2
Selling, general and administrative | Eisai | ADUHELM
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Expense incurred by the collaboration				125.6	45.2		237.4	67.9
Expense reflected within statements of income				67.6	25.2		127.9	37.5
Selling, general and administrative | Sage Therapeutics Inc.
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Expense incurred by the collaboration				10.5	0.0		15.8	0.0
Expense reflected within statements of income				$ 5.2	$ 0.0		$ 7.9	$ 0.0