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Summary of Significant Accounting Policies - Narrative (Details) $ in Millions | 9 Months Ended | 12 Months Ended | 24 Months Ended |
--- | --- | --- | --- | ---
Dec. 31, 2018 USD ($) | Dec. 31, 2020 USD ($) product segment | Dec. 31, 2019 USD ($) product | Dec. 31, 2018 USD ($) | Dec. 31, 2020 USD ($) | Dec. 31, 2017 USD ($)
Property, Plant and Equipment [Line Items] |  |  |  |  |  |
Number of ophthalmolgy biosimilar products | product |  | 2 | 2 |  |  |
Interest in subsidiary (less than given percentage) |  | 100.00% |  |  |  |
Operating lease assets |  | $ 433.3 | $ 427.0 |  | $ 433.3 |
Present value of operating lease liabilities |  | $ 485.2 | 486.3 |  | 485.2 |
Number of operating segments | segment |  | 1 |  |  |  |
Advertising costs |  | $ 111.8 | 79.2 | $ 90.2 |  |
Stockholders equity | $ 13,031.6 | 10,686.1 | $ 13,339.1 | 13,031.6 | 10,686.1 | $ 12,598.1
Eisai |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |
Capitalized inventory |  | $ 93.8 |  |  | $ 93.8 |
Percentage of future development costs related to Eisai | 15.00% | 45.00% |  |  | 45.00% |
Cumulative Effect, Period of Adoption, Adjustment |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |
Operating lease assets | $ 463.0 |  |  | 463.0 |  |
Present value of operating lease liabilities | 526.0 |  |  | 526.0 |  |
Stockholders equity |  |  |  |  |  | 106.1
Minimum |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |
Payment terms of accounts receivable arising from product sales |  | 30 days |  |  |  |
Minimum | Computer Software and Hardware |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment, useful life |  | 3 years |  |  |  |
Maximum |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |
Payment terms of accounts receivable arising from product sales |  | 90 days |  |  |  |
Maximum | Computer Software and Hardware |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment, useful life |  | 5 years |  |  |  |
Deferred Income Tax Charge | Cumulative Effect, Period of Adoption, Adjustment |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |
Stockholders equity |  |  |  |  |  | 500.0
Retained Earnings | Cumulative Effect, Period of Adoption, Adjustment |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |
Stockholders equity |  |  |  |  |  | $ 500.0
Deferred tax liability | Cumulative Effect, Period of Adoption, Adjustment |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |
Stockholders equity | $ 400.0 |  |  | $ 400.0 |  |

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