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Significant Accounting Policies, Basic and Diluted Net Income (Loss) per Share (Details) - USD ($)
$ / shares in Units, shares in Thousands, $ in Thousands
3 Months Ended 7 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Jul. 18, 2017
Dec. 31, 2017
Basic Net Income (Loss) per Share [Abstract]        
Basic net income (loss) per share (in dollars per share) $ (0.01) $ 0.07    
Net income (loss) $ (1,420) $ 8,964    
Net income (loss) available to Ionis common shareholders $ (1,212) $ 8,964    
Shares used in computing basic net income (loss) per share (in shares) 125,330 122,861    
Income available to Ionis common shareholders, plus assumed conversions   $ 8,964    
Shares issuable related to our ESPP   60    
Shares used in computing diluted net income per share (in shares) 125,330 124,972    
Diluted net income per share (in dollars per share) $ (0.01) $ 0.07    
1 Percent Convertible Senior Notes [Member]        
Basic Net Income (Loss) per Share [Abstract]        
Interest rate on convertible senior notes 1.00% 1.00%   1.00%
2 3/4 Percent Convertible Senior Notes [Member]        
Basic Net Income (Loss) per Share [Abstract]        
Interest rate on convertible senior notes   2.75%    
Stock Options [Member]        
Basic Net Income (Loss) per Share [Abstract]        
Shares issuable related to stock-based compensation (in shares)   1,674    
Restricted Stock Awards [Member]        
Basic Net Income (Loss) per Share [Abstract]        
Shares issuable related to stock-based compensation (in shares)   377    
Ionis [Member]        
Basic Net Income (Loss) per Share [Abstract]        
Net income (loss) $ 18,785      
Akcea [Member]        
Basic Net Income (Loss) per Share [Abstract]        
Net income (loss) $ (19,997)      
Percentage ownership   100.00% 100.00%  
Akcea [Member] | Common Stock [Member]        
Basic Net Income (Loss) per Share [Abstract]        
Weighted average shares owned in Akcea (in shares) 45,448      
Basic net income (loss) per share (in dollars per share) $ (0.44)      
Net income (loss) $ (19,997)