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Quarterly Financial Data (Unaudited) (Details Textual) - USD ($) shares in Millions, $ in Millions | 1 Months Ended | 3 Months Ended | 9 Months Ended | 12 Months Ended |
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May 31, 2018 | Oct. 31, 2017 | Dec. 31, 2019 | Sep. 30, 2019 | Jun. 30, 2019 | Mar. 31, 2019 | Dec. 31, 2018 | Sep. 30, 2018 | Jun. 30, 2018 | Mar. 31, 2018 | Dec. 31, 2017 | Jun. 30, 2017 | Mar. 31, 2017 | Dec. 31, 2013 | Sep. 30, 2019 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2015 | Jun. 05, 2018
Schedule Of Development Milestone And Collaboration [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Unrealized gain (loss) on strategic investments |  |  | $ 2.8 | $ (4.6) | $ (174.2) | $ 376.1 | $ (12.2) | $ 141.2 | $ 5.4 | $ (6.4) |  |  |  |  |  |  |  |  |  |
GILTI |  |  | (1,381.6) |  |  |  | (544.6) |  |  |  |  |  |  |  |  | $ (1,381.6) | $ (544.6) |  |  |
Loss (gain) on fair value remeasurement of contingent consideration |  |  | (2.6) | 57.8 | $ 20.0 | $ (11.5) | (79.3) | 87.9 | (1.9) | 5.6 |  |  |  |  |  | 63.7 | 12.3 | $ (62.7) |  |
Research and development |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2,280.6 | 2,597.2 | 2,253.6 |  |
Acquired in-process research and development |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 0.0 | 112.5 | 120.0 |  |
Payments to Noncontrolling Interests |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | (4.3) | 36.4 | 134.1 |  |
Loss on divestiture of Hillerød, Denmark manufacturing operations |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 55.3 | 0.0 | 0.0 |  |
Net income (loss) attributable to noncontrolling interests, net of tax |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 0.0 | 43.3 | 131.0 |  |
Income tax benefit (expense) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1,158.0 | 1,425.6 | 2,458.7 |  |
Samsung Bioepis |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule Of Development Milestone And Collaboration [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Research and development expense on 2018 Ionis Agreement |  |  | 63.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Research and development |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 46.0 |  |  |  |  |  |
Upfront And Milestone Payments Made To Collaborative Partner |  |  | 100.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Bristol-Myers Squibb |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule Of Development Milestone And Collaboration [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Research and development |  |  |  |  |  |  |  |  |  |  |  | $ 300.0 |  |  |  | 144.0 | 97.0 |  |  |
iPerian |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule Of Development Milestone And Collaboration [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Developmental Milestone Payment |  |  |  |  |  |  |  |  |  |  |  | 60.0 |  |  |  |  |  |  |  |
Alkermes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule Of Development Milestone And Collaboration [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Research and development |  |  |  |  |  |  |  |  |  |  | $ 50.0 |  |  |  |  | 53.5 | 68.7 | 80.3 |  |
Upfront And Milestone Payments Made To Collaborative Partner |  |  |  |  |  |  |  |  |  |  | 28.0 |  |  |  |  |  |  |  |  |
Ionis Pharmaceuticals |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule Of Development Milestone And Collaboration [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Research and development expense on 2018 Ionis Agreement |  |  | 45.0 |  |  |  |  |  | 486.2 |  |  |  |  |  |  |  |  |  |  |
Research and development |  |  |  |  |  |  |  |  | 324.1 |  |  |  |  |  |  | 30.0 |  |  | $ 10.0 |
Premium on purchase of common stock |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 162.1
Investment in common stock, shares purchased |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 11.5
Upfront And Milestone Payments Made To Collaborative Partner |  |  |  |  |  |  |  |  |  |  | 25.0 |  |  |  |  |  |  |  |  |
Neurimmune |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule Of Development Milestone And Collaboration [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payments to Noncontrolling Interests | $ 50.0 | $ 150.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Additional reduction in royalty rate payable on commercial sales | 5.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Reduction in royalty rate payable on commercial sales |  | 15.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
TECFIDERA |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule Of Development Milestone And Collaboration [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Impairment of in-licensed patent |  |  |  |  |  |  | 176.8 |  |  |  |  |  | $ 328.2 |  |  |  |  |  |  |
Karyopharm |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule Of Development Milestone And Collaboration [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acquired in-process research and development |  |  |  |  |  |  |  |  |  | $ 10.0 |  |  |  |  |  |  |  |  |  |
Remedy Pharmaceutical |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule Of Development Milestone And Collaboration [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acquired in-process research and development |  |  |  |  |  |  |  |  |  |  |  | $ 120.0 |  |  |  |  |  |  |  |
Pfizer |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule Of Development Milestone And Collaboration [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acquired in-process research and development |  |  |  |  |  |  |  |  | $ 75.0 |  |  |  |  |  |  |  |  |  |  |
AliveGen |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule Of Development Milestone And Collaboration [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acquired in-process research and development |  |  |  |  |  |  |  | 27.5 |  |  |  |  |  |  |  |  |  |  |  |
In-process research and development |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule Of Development Milestone And Collaboration [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Impairment of IPR&D |  |  |  | $ 215.9 |  |  |  | $ 189.3 |  |  |  |  |  |  |  |  |  |  |  |
2017 Tax Act |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule Of Development Milestone And Collaboration [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
GILTI |  |  |  |  |  |  | $ 135.8 |  |  |  |  |  |  |  |  |  | $ 135.8 |  |  |
Income tax benefit (expense) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1,200.0 |  |
Transition toll tax liabilities |  |  | 697.0 |  |  |  |  |  |  |  | $ 989.6 |  |  |  |  | 697.0 |  | $ 989.6 |  |
Denmark Manufacturing Operations |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule Of Development Milestone And Collaboration [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss on divestiture of Hillerød, Denmark manufacturing operations, net of tax |  |  | $ 40.2 |  |  |  |  |  |  |  |  |  |  |  | $ 164.4 | 124.2 |  |  |  |
Loss on divestiture of Hillerød, Denmark manufacturing operations |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 95.5 | 55.3 |  |  |  |
Tax expense on disposal group |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 68.9 |  |  |  |

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