Segment Information - Schedule of Significant Segment Expenses (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2025	Mar. 31, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Segment Reporting Information [Line Items]
Total revenues	$ 3,076.4		$ 2,771.9	$ 8,811.3	$ 8,108.1
Cost of sales	414.8		392.6	1,185.3	1,107.1
Acquired in-process research and development expenses	54.5		15.0	76.5	4,540.9
Intangible asset impairment charge	0.0	$ 379.0	0.0	379.0	0.0
Interest income	(125.7)		(132.2)	(369.0)	(469.9)
Provision for income taxes	215.9		178.7	550.1	560.6
Net income (loss)	1,082.9		1,045.4	2,762.1	(1,448.6)
Pharmaceuticals
Segment Reporting Information [Line Items]
Total revenues	3,076.4		2,771.9	8,811.3	8,108.1
Research expenses	210.0		193.8	625.4	597.2
Development expenses	767.7		682.1	2,310.4	2,034.4
Acquired in-process research and development expenses	54.5		15.0	76.5	4,540.9
Selling and other commercial expenses	278.3		216.9	784.0	604.9
General and administrative expenses	166.8		154.9	482.1	481.8
Intangible asset impairment charge	0.0		0.0	379.0	0.0
Interest income	(125.7)		(132.2)	(369.0)	(469.9)
Other segment items	11.2		24.7	25.4	99.7
Provision for income taxes	215.9		178.7	550.1	560.6
Net income (loss)	1,082.9		1,045.4	2,762.1	(1,448.6)
Products
Segment Reporting Information [Line Items]
Total revenues	3,076.4		2,771.9	8,780.6	8,108.1
Products | Pharmaceuticals
Segment Reporting Information [Line Items]
Cost of sales	139.6		131.6	410.7	363.9
Royalty | Pharmaceuticals
Segment Reporting Information [Line Items]
Cost of sales	$ 275.2		$ 261.0	$ 774.6	$ 743.2