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Fair Value Measurements (Details Textual) - USD ($) $ in Millions | 1 Months Ended | 3 Months Ended | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | --- | ---
Feb. 28, 2015 | Mar. 31, 2012 | Dec. 31, 2010 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Feb. 12, 2015 | Mar. 04, 2008
Business Acquisition, Contingent Consideration [Line Items] |  |  |  |  |  |  |  |
Fair value measurements, changes in valuation techniques |  |  |  | 0 | 0 |  |  |
Maximum contingent consideration in the form of development and approval milestones |  |  |  | $ 1,100.0 |  |  |  |
Convergence Pharmaceuticals |  |  |  |  |  |  |  |
Business Acquisition, Contingent Consideration [Line Items] |  |  |  |  |  |  |  |
Milestone converted to short-term obligation |  |  |  |  | $ 7.9 |  |  |
Additions | $ 274.5 |  |  |  |  |  |  |
Contingent consideration obligations |  |  |  | 259.0 | 258.9 |  |  |
Maximum contingent consideration in the form of development and approval milestones |  |  |  | $ 400.0 |  |  | $ 450.0 |
Discount rate used for net cash outflow projections for fair value measurement | 2.00% |  |  | 2.40% |  |  |  |
In-process research and development |  |  |  |  |  |  | $ 424.6 |
Stromedix, Inc. |  |  |  |  |  |  |  |
Business Acquisition, Contingent Consideration [Line Items] |  |  |  |  |  |  |  |
Additions |  | $ 122.2 |  |  |  |  |  |
Contingent consideration obligations |  |  |  | $ 162.4 | 133.2 |  |  |
Maximum contingent consideration in the form of development and approval milestones |  |  |  | $ 344.0 |  |  |  |
Discount rate used for net cash outflow projections for fair value measurement |  |  |  | 2.40% |  |  |  |
Biogen Idec International Neuroscience GmbH |  |  |  |  |  |  |  |
Business Acquisition, Contingent Consideration [Line Items] |  |  |  |  |  |  |  |
Additions |  |  | $ 81.2 |  |  |  |  |
Contingent consideration obligations |  |  |  | $ 102.2 | 75.5 |  |  |
Maximum contingent consideration in the form of development and approval milestones |  |  |  | $ 355.0 |  |  |  |
Discount rate used for net cash outflow projections for fair value measurement |  |  |  | 2.80% |  |  |  |
Milestone payments made during period |  |  |  | $ 6.7 |  |  |  |
6.875% Senior Notes due 2018 |  |  |  |  |  |  |  |
Business Acquisition, Contingent Consideration [Line Items] |  |  |  |  |  |  |  |
Interest rate on senior notes |  |  |  | 6.875% |  |  |  | 6.875%
6.875% Senior Notes aggregate principal amount |  |  |  |  |  |  |  | $ 550.0
2.90% Senior Notes due 2020 |  |  |  |  |  |  |  |
Business Acquisition, Contingent Consideration [Line Items] |  |  |  |  |  |  |  |
Interest rate on senior notes |  |  |  | 2.90% |  | 2.90% |  |
6.875% Senior Notes aggregate principal amount |  |  |  |  |  | $ 1,500.0 |  |
3.625% Senior Notes due 2022 |  |  |  |  |  |  |  |
Business Acquisition, Contingent Consideration [Line Items] |  |  |  |  |  |  |  |
Interest rate on senior notes |  |  |  | 3.625% |  | 3.625% |  |
6.875% Senior Notes aggregate principal amount |  |  |  |  |  | $ 1,000.0 |  |
4.05% Senior Notes due 2025 |  |  |  |  |  |  |  |
Business Acquisition, Contingent Consideration [Line Items] |  |  |  |  |  |  |  |
Interest rate on senior notes |  |  |  | 4.05% |  | 4.05% |  |
6.875% Senior Notes aggregate principal amount |  |  |  |  |  | $ 1,750.0 |  |
5.20% Senior Notes due 2045 |  |  |  |  |  |  |  |
Business Acquisition, Contingent Consideration [Line Items] |  |  |  |  |  |  |  |
Interest rate on senior notes |  |  |  | 5.20% |  | 5.20% |  |
6.875% Senior Notes aggregate principal amount |  |  |  |  |  | $ 1,750.0 |  |
Other long-term liabilities |  |  |  |  |  |  |  |
Business Acquisition, Contingent Consideration [Line Items] |  |  |  |  |  |  |  |
Contingent consideration obligations |  |  |  | $ 279.0 | $ 246.8 |  |  |
Accrued expenses and other | Convergence Pharmaceuticals |  |  |  |  |  |  |  |
Business Acquisition, Contingent Consideration [Line Items] |  |  |  |  |  |  |  |
Contingent consideration obligations |  |  |  | 147.9 |  |  |  |
Accrued expenses and other | Stromedix, Inc. |  |  |  |  |  |  |  |
Business Acquisition, Contingent Consideration [Line Items] |  |  |  |  |  |  |  |
Contingent consideration obligations |  |  |  | 76.7 |  |  |  |
Accrued expenses and other | Biogen Idec International Neuroscience GmbH |  |  |  |  |  |  |  |
Business Acquisition, Contingent Consideration [Line Items] |  |  |  |  |  |  |  |
Contingent consideration obligations |  |  |  | $ 20.0 |  |  |  |

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