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Organization and Significant Accounting Policies, Basic Net Income (Loss) per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands | 1 Months Ended | 2 Months Ended | 3 Months Ended | 10 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | --- | ---
Oct. 31, 2020 | Dec. 31, 2020 | Dec. 31, 2021 | Dec. 31, 2020 | Oct. 31, 2020 | Dec. 31, 2021 | Dec. 31, 2020 | Dec. 31, 2019 | Sep. 30, 2020
Basic Net Income (Loss) per Share [Abstract] |  |  |  |  |  |  |  |  |  |  |
Basic net income (loss) per share (in dollars per share) |  |  | $ 1.59 | [1],[2] | $ (2.54) | [1],[2] |  | $ (0.20) | $ (3.18) | $ 2.00 |
Net loss |  |  | $ 224,613 |  | $ (354,532) |  |  | $ (28,597) | $ (444,263) | $ 278,143 |
Income (loss) available to Ionis common stockholders |  |  | $ 224,612 |  | $ (356,008) |  |  |  | $ (444,464) | $ 280,560 |
Weighted average shares outstanding (in shares) |  |  | 141,205 |  | 139,956 |  |  | 141,021 | 139,612 | 139,998 |
Ionis [Member] |  |  |  |  |  |  |  |  |  |  |
Basic Net Income (Loss) per Share [Abstract] |  |  |  |  |  |  |  |  |  |  |
Net loss |  |  |  |  | $ (266,418) |  |  |  | $ (246,702) | $ 246,487 |
Akcea [Member] |  |  |  |  |  |  |  |  |  |  |
Basic Net Income (Loss) per Share [Abstract] |  |  |  |  |  |  |  |  |  |  |
Percentage ownership | 100.00% |  | 100.00% |  |  |  | 100.00% | 100.00% |  |  | 76.00%
Basic Net Income (Loss) per Share [Abstract] |  |  |  |  |  |  |  |  |  |  |
Weighted average shares owned in Akcea (in shares) |  |  |  |  | 77,095 |  |  |  |  |  |
Basic net income (loss) per share (in dollars per share) |  |  |  |  | $ (0.05) |  |  |  |  |  |
Net loss | $ (3,603) | $ (85,987) |  |  | $ (89,590) |  |  |  |  |  |
Akcea [Member] |  |  |  |  |  |  |  |  |  |  |
Basic Net Income (Loss) per Share [Abstract] |  |  |  |  |  |  |  |  |  |  |
Weighted average shares owned in Akcea (in shares) |  |  |  |  |  |  | 77,095 |  |  | 70,100 |
Basic net income (loss) per share (in dollars per share) |  |  |  |  |  |  | $ (1.45) |  |  | $ 0.49 |
Net loss |  | $ (85,987) |  |  |  |  | $ (111,775) |  | $ (197,762) | $ 34,073 |
[1] | We compute net income (loss) per share independently for each quarter during the year.
[2] | As discussed in Note 1, Organization and Significant Accounting Policies, we compute basic net income (loss) per share by dividing the total net income (loss) attributable to our common stockholders by our weighted-average number of common shares outstanding during the period. Our basic net income per share for the fourth quarter of 2021 was $1.59.Our basic net loss per share calculation for the fourth quarter of 2020 considered our net loss for Ionis on a stand-alone basis plus our share of Akcea’s net loss for the period. To calculate the portion of Akcea’s net loss attributable to our ownership, we multiplied Akcea’s loss per share by the weighted average shares we owned in Akcea during the period. As a result of this calculation, our total net loss available to Ionis common stockholders for the calculation of net loss per share is different than net loss attributable to Ionis Pharmaceuticals, Inc. common stockholders in the consolidated statements of operations.

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