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Quarterly Financial Data (Unaudited) (Details Textual) - USD ($) $ in Millions | 1 Months Ended | 3 Months Ended | 12 Months Ended |
--- | --- | --- | --- | ---
Oct. 31, 2017 | Dec. 31, 2017 | Sep. 30, 2017 | Jun. 30, 2017 | Mar. 31, 2017 | Dec. 31, 2016 | Sep. 30, 2016 | Jun. 30, 2016 | Sep. 30, 2013 | Mar. 31, 2012 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2012
Schedule Of Development Milestone And Collaboration [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amortization of Intangible Assets |  |  |  |  |  |  |  |  |  |  | $ 814.7 | $ 385.6 | $ 382.6 |
Research and development |  |  |  |  |  |  |  |  |  |  | 2,253.6 | 1,973.3 | 2,012.8 |
Acquired in-process research and development |  |  |  |  |  |  |  |  |  |  | 120.0 | 0.0 | 0.0 |
Net income (loss) attributable to noncontrolling interests, net of tax |  |  |  |  |  |  |  |  |  |  | 131.0 | (7.1) | 46.2 |
Income tax benefit (expense) |  |  |  |  |  |  |  |  |  |  | 2,458.7 | 1,237.3 | 1,161.6 |
TECFIDERA litigation settlement charge |  |  |  |  |  | $ 454.8 |  |  |  |  | 0.0 | 454.8 | 0.0 |
Bristol-Myers Squibb [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule Of Development Milestone And Collaboration [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Research and development |  |  |  | $ 300.0 |  |  |  |  |  |  |  |  |  |
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 |  |  |  |  |  |  |  |  | 80.3 |  |  |
Upfront And Milestone Payments Made To Collaborative Partner |  | 28.0 |  |  |  |  |  |  |  |  | 28.0 |  |  |
Ionis Pharmaceuticals, Inc. |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule Of Development Milestone And Collaboration [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Research and development |  |  |  |  |  |  |  |  | $ 75.0 |  | 12.0 |  | 10.0 |
Upfront And Milestone Payments Made To Collaborative Partner |  | 25.0 |  |  |  |  |  |  | $ 100.0 |  | 25.0 |  |  | $ 30.0
License Fee |  |  |  |  |  |  |  |  |  |  | 70.0 |  |  |
Eisai |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule Of Development Milestone And Collaboration [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Research and development |  |  |  |  |  |  |  |  |  |  | 263.4 | 50.0 |  |
Neurimmune |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule Of Development Milestone And Collaboration [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net income (loss) attributable to noncontrolling interests, net of tax |  | 150.0 |  |  |  |  |  |  |  |  |  |  |  |
Reduction in royalty rate payable on commercial sales | 15.00% |  |  |  |  |  |  |  |  |  |  |  |  |
Cambridge, MA |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule Of Development Milestone And Collaboration [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accelerated depreciation |  |  |  |  |  | $ 14.0 | $ 15.7 | $ 15.8 |  |  |  | 45.5 |  |
TECFIDERA |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule Of Development Milestone And Collaboration [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amortization of Intangible Assets |  | $ 30.8 | $ 30.4 | 29.4 | $ 353.6 |  |  |  |  |  | $ 444.2 |  |  |
SPINRAZA | Ionis Pharmaceuticals, Inc. |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule Of Development Milestone And Collaboration [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Research and development |  |  |  |  |  |  |  |  |  | $ 29.0 |  | $ 35.3 | $ 42.8 |
License Fee |  |  |  |  |  |  | $ 75.0 |  |  |  |  |  |  |
Remedy Pharmaceutical [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule Of Development Milestone And Collaboration [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acquired in-process research and development |  |  |  | $ 120.0 |  |  |  |  |  |  |  |  |  |

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