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Fair Value Measurements, Fair Value Measurements Using Significant Unobservable Inputs (Level 3) (Details) - USD ($) $ in Thousands | 9 Months Ended |  |  |
--- | --- | --- | --- | ---
Sep. 30, 2017 | Sep. 30, 2018 | Dec. 31, 2017 | Jan. 31, 2017
Novartis [Member] |  |  |  |
Novartis Future Stock Purchase [Abstract] |  |  |  |
Fair value of potential future premium |  |  |  | $ 5,000
1 Percent Convertible Senior Notes [Member] |  |  |  |
Fair Value Measurements [Abstract] |  |  |  |
Interest rate on convertible senior notes | 1.00% | 1.00% | 1.00% |
Significant Other Observable Inputs (Level 2) [Member] | 1 Percent Convertible Senior Notes [Member] |  |  |  |
Fair Value Measurements [Abstract] |  |  |  |
Fair value of convertible notes |  | $ 725,700 |  |
Significant Unobservable Inputs (Level 3) [Member] |  |  |  |
Fair Value Measurements [Abstract] |  |  |  |
Financial instruments |  | $ 0 | $ 0 |
Akcea [Member] | Novartis [Member] |  |  |  |
Novartis Future Stock Purchase [Abstract] |  |  |  |
Additional amount of common stock required to be purchased |  |  |  | $ 50,000
Potential Future Premium [Member] |  |  |  |
Unobservable Inputs (Level 3) [Roll Forward] |  |  |  |
Beginning balance of Level 3 instruments | $ 0 |  |  |
Value of the potential premium we would have received from Novartis at inception of the SPA (January 2017) | 5,035 |  |  |
Recurring fair value adjustment during the nine months ended September 30, 2017 | (5,035) |  |  |
Ending balance of Level 3 instruments | $ 0 |  |  |

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- Definition The fair value of the potential future premium the Company will receive if Novartis purchases its common stock under a Stock Purchase Agreement (SPA) entered into in January 2017. + References No definition available. + Details
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- Definition Contractual interest rate for funds borrowed, under the debt agreement. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22(a)(1)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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