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Acquisitions - Narrative (Details) $ / shares in Units, $ in Millions |  |  | 3 Months Ended | 9 Months Ended | 12 Months Ended
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Jul. 02, 2024 USD ($) | Sep. 26, 2023 USD ($) ft² $ / shares | Dec. 31, 2024 USD ($) | Sep. 30, 2024 USD ($) | Sep. 30, 2023 USD ($) | Sep. 30, 2024 USD ($) | Sep. 30, 2023 USD ($) | Dec. 31, 2023 USD ($)
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Share-based compensation expense |  |  |  | $ (75.0) | $ (64.8) | $ (224.0) | $ (217.8) |
Current portion of contingent consideration obligations |  |  |  | 287.1 |  | 287.1 |  | $ 0.0
Noncurrent portion of contingent consideration obligations |  |  |  | 221.8 |  | 221.8 |  |
Goodwill |  |  |  | 6,485.8 |  | 6,485.8 |  | 6,219.2
Selling, general and administrative |  |  |  | 588.4 | 788.2 | 1,723.7 | 1,941.2 |
Net income |  |  |  | 388.5 | (68.3) | 1,365.5 | 912.6 |
Cost of sales, excluding amortization and impairment of acquired intangible assets |  |  |  | 638.7 | 659.6 | 1,726.9 | 1,915.1 |
Felzartamab And Izastobart/HIB210 | Milestone Payment, Future Development | Human Immunology Biosciences Pre-Existing In License Commitments |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Contingent consideration obligations | $ 130.0 |  |  |  |  |  |  |
Felzartamab And Izastobart/HIB210 | Milestone Payment, Regulatory | Human Immunology Biosciences Pre-Existing In License Commitments |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Contingent consideration obligations | 230.0 |  |  |  |  |  |  |
Felzartamab And Izastobart/HIB210 | Milestone Payment, Commercial | Human Immunology Biosciences Pre-Existing In License Commitments |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Contingent consideration obligations | 640.0 |  |  |  |  |  |  |
Forecast |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Outstanding equity awards |  |  | $ 983.9 |  |  |  |  |
Research and development |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Share-based compensation expense |  |  |  | 69.8 | 217.1 | 124.5 | 276.3 |
Selling, general and administrative |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Share-based compensation expense |  |  |  | 61.3 | 234.9 | 150.9 | 333.4 |
Human Immunology Biosciences |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Total transaction value | 1,137.3 |  |  |  |  |  |  |
Range of outcomes, value, high | 650.0 |  |  |  |  |  |  |
Non-vested equity awards | 74.5 |  |  |  |  |  |  |
Share-based compensation expense | 56.4 |  |  | 56.4 | 0.0 | 56.4 | 0.0 |
Working capital adjustment | 43.7 |  |  |  |  |  |  |
Contingent consideration obligations | 485.1 |  |  |  |  |  |  |
Current portion of contingent consideration obligations | 279.3 |  |  |  |  |  |  |
Noncurrent portion of contingent consideration obligations | 205.8 |  |  |  |  |  |  |
Intangible assets: | 1,600.0 |  |  |  |  |  |  |
Goodwill | 256.5 |  |  |  |  |  |  |
Acquisition related costs |  |  |  | 2.8 |  | 2.8 |  |
Operating lease liabilities | 1.2 |  |  |  |  |  |  |
Operating lease assets | $ 1.2 |  |  |  |  |  |  |
Human Immunology Biosciences | Discount rate |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Measurement input | 0.145 |  |  |  |  |  |  |
Human Immunology Biosciences | Measurement Input, Probability Rate |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Discount rate | 0.670 |  |  |  |  |  |  |
Human Immunology Biosciences | Previously Reported |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Total transaction value | $ 1,150.0 |  |  |  |  |  |  |
Human Immunology Biosciences | Minimum | Discount rate |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Discount rate | 0.062 |  |  |  |  |  |  |
Human Immunology Biosciences | Maximum | Discount rate |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Discount rate | 0.070 |  |  |  |  |  |  |
Human Immunology Biosciences | Maximum | Phase 3 Clinical Trial, Felzartamab One |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Current portion of contingent consideration obligations | $ 150.0 |  |  |  |  |  |  |
Human Immunology Biosciences | Maximum | Phase 3 Clinical Trial, Felzartamab Two |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Current portion of contingent consideration obligations | 150.0 |  |  |  |  |  |  |
Human Immunology Biosciences | Research and development |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Share-based compensation expense | 42.5 |  |  |  |  |  |  |
Human Immunology Biosciences | Selling, general and administrative |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Share-based compensation expense | 13.9 |  |  |  |  |  |  |
Human Immunology Biosciences | In-process research and development |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Intangible assets: |  |  |  | 1,600.0 |  | 1,600.0 |  |
Human Immunology Biosciences | Other clinical programs |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Intangible assets: | $ 7.9 |  |  |  |  |  |  |
Reata Pharmaceuticals, Inc |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Total transaction value |  | $ 6,602.9 |  |  |  |  |  |
Share-based compensation expense |  |  |  | 0.0 | $ 387.0 | 0.0 | $ 387.0 |
Goodwill |  | $ 469.2 |  | 469.2 |  | 469.2 |  |
Price per share (in dollars per share) | $ / shares |  | $ 172.50 |  |  |  |  |  |
Equity compensation pre-acquisition services |  | $ 590.5 |  |  |  |  |  |
Operating expenses |  | 393.4 |  |  |  |  |  |
Selling, general and administrative |  | 196.4 |  |  |  |  |  |
Research and development expense asset acquired |  | 197.0 |  |  |  |  |  |
Inventory |  | $ 1,300.0 |  | 1,259.0 |  | 1,259.0 |  |
Inventory amortization period |  | 4 years |  |  |  |  |  |
Intangible assets discount rate |  | 14.30% |  |  |  |  |  |
Operating lease area | ft² |  | 327,400 |  |  |  |  |  |
Lease term |  | 16 years |  |  |  |  |  |
Operating lease liabilities |  | $ 151.8 |  | 151.8 |  | 151.8 |  |
Remaining lease term |  | 15 years |  |  |  |  |  |
Operating lease assets |  | $ 121.2 |  | 121.2 |  | 121.2 |  |
Reata Pharmaceuticals, Inc | Fair Value Adjustment to Inventory |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Net income |  |  |  | 50.9 |  | 185.1 |  |
Reata Pharmaceuticals, Inc | Clinical Inventory | Fair Value Adjustment to Inventory |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Cost of sales, excluding amortization and impairment of acquired intangible assets |  |  |  | 2.5 |  | 48.5 |  |
Reata Pharmaceuticals, Inc | Secured Debt | The Credit Facility | Line of Credit |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Long-term debt, gross |  | 1,000.0 |  |  |  |  |  |
Reata Pharmaceuticals, Inc | Selling, general and administrative |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Acquisition cost expensed |  |  |  |  |  |  |  | $ 26.3
Reata Pharmaceuticals, Inc | Completed technology for SKYCLARYS (U.S.) |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Intangible assets: |  | 4,200.0 |  | 4,200.0 |  | 4,200.0 |  |
Reata Pharmaceuticals, Inc | In-process research and development |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Intangible assets: |  | 2,300.0 |  | 2,300.0 |  | 2,300.0 |  |
Reata Pharmaceuticals, Inc | Priority review voucher |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Intangible assets: |  | 100.0 |  | 100.0 |  | 100.0 |  |
Reata Pharmaceuticals, Inc | Other clinical programs |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Intangible assets: |  | $ 40.0 |  | $ 40.0 |  | $ 40.0 |  |

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