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Commitments and Contingencies (Details Textual) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | ---
Dec. 31, 2017 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2006 | Sep. 30, 2017 | Jun. 30, 2017 | Mar. 31, 2017 | Jun. 30, 2013 | Apr. 02, 2013
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Maximum contingent consideration in the form of development and approval milestones | $ 1,100.0 | $ 1,100.0 |  |  |  |  |  |  |  |
Increase to goodwill |  | 1,267.3 | $ 1,026.9 |  |  |  |  |  |  |
Potential future milestone payments commitment, approximately | 4,200.0 | 4,200.0 |  |  |  |  |  |  |  |
Cancellable future commitments | 460.0 | 460.0 |  |  |  |  |  |  |  |
Commitments And Contingencies (Textual) |  |  |  |  |  |  |  |  |  |
Accrued expenses | 40.0 | 40.0 |  |  |  |  |  |  |  |
Liabilities associated with uncertain tax positions | 77.3 | 77.3 |  |  |  |  |  |  |  |
Lease rent expense which terminates at various dates through 2028 |  | 65.3 | 68.7 | $ 68.6 |  |  |  |  |  |
TYSABRI |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Future contingent payment for annual worldwide net sales up to $2.0 billion |  |  |  |  |  |  |  |  |  | 18.00%
Future contingent payment threshold |  |  |  |  |  |  |  |  |  | $ 2,000.0
Future contingent payment for annual worldwide net sales that exceed $2.0 billion |  |  |  |  |  |  |  |  |  | 25.00%
Fumapharm AG |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Amount paid in cash |  |  |  |  | $ 220.0 |  |  |  |  |
Solothurn |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Cancellable future commitments | 270.0 | 270.0 |  |  |  |  |  |  |  |
TECFIDERA | Fumapharm AG |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Maximum contingent consideration in the form of development and approval milestones |  |  |  |  |  |  |  |  | $ 15.0 |
Increase to goodwill |  | 1,500.0 | 1,200.0 |  |  |  |  |  |  |
Fourteen billion | TECFIDERA | Fumapharm AG |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Increase to goodwill |  | 1,200.0 |  |  |  |  |  |  |  |
Cumulative sales level |  |  |  |  |  | $ 14,000.0 |  |  |  |
Eleven billion | TECFIDERA | Fumapharm AG |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative sales level |  |  | $ 11,000.0 |  |  |  |  |  |  |
Twelve billion | TECFIDERA | Fumapharm AG |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative sales level |  |  |  |  |  |  |  | $ 12,000.0 |  |
Thirteen billion | TECFIDERA | Fumapharm AG |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative sales level |  |  |  |  |  |  | $ 13,000.0 |  |  |
Fifteen billion | TECFIDERA | Fumapharm AG |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative sales level | 15,000.0 | 15,000.0 |  |  |  |  |  |  |  |
Sixteen billion | TECFIDERA | Fumapharm AG |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Increase to goodwill | 600.0 |  |  |  |  |  |  |  |  |
Cumulative sales level | 16,000.0 | 16,000.0 |  |  |  |  |  |  |  |
One billion | TECFIDERA | Fumapharm AG |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative sales level | 1,000.0 | 1,000.0 |  |  |  |  |  |  |  |
Each additional one billion up to twenty billion | TECFIDERA | Fumapharm AG |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Increase to goodwill |  | 300.0 |  |  |  |  |  |  |  |
Cumulative sales level | 20,000.0 | 20,000.0 |  |  |  |  |  |  |  |
Three billion | TECFIDERA | Fumapharm AG |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative sales level | 3,000.0 | 3,000.0 |  |  |  |  |  |  |  |
Development Milestones [Member] |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Potential future milestone payments commitment, approximately | 700.0 | 700.0 |  |  |  |  |  |  |  |
Regulatory Milestones [Member] |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Potential future milestone payments commitment, approximately | 1,500.0 | 1,500.0 |  |  |  |  |  |  |  |
Commercial Milestones [Member] |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Potential future milestone payments commitment, approximately | 2,000.0 | 2,000.0 |  |  |  |  |  |  |  |
2017 Tax Act |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Accrued income tax liability under the Transition Toll Tax | 989.6 | 989.6 |  |  |  |  |  |  |  |
Accrued income tax liability under the Transition Toll Tax, current | $ 78.3 | $ 78.3 |  |  |  |  |  |  |  |

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