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Organization and Significant Accounting Policies, Impact of Recently Issued Accounting Standards (Details) - USD ($)
$ in Thousands
3 Months Ended 12 Months Ended
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2016
Sep. 30, 2016
Jun. 30, 2016
Mar. 31, 2016
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Impact of Recently Issued Accounting Standards [Abstract]                      
Revenue $ 172,299 $ 120,911 $ 104,152 $ 110,304 $ 160,349 $ 110,927 $ 38,470 $ 36,874 $ 507,666 $ 346,620 $ 283,703
Accumulated deficit (1,187,398)       (1,181,428)       (1,187,398) (1,181,428)  
Income tax expense                 (5,980) 2,934 372
Biogen [Member]                      
Impact of Recently Issued Accounting Standards [Abstract]                      
Milestone payment earned                 50,000    
Revenue                 259,800 207,900 $ 106,200
Deferred revenue 69,300       $ 67,500       69,300 67,500  
Biogen [Member] | IONIS-MAPT [Member]                      
Impact of Recently Issued Accounting Standards [Abstract]                      
Milestone payment earned                 10,000    
Revenue                 10,000    
Recognition of Revenue from Customers [Member] | Plan [Member]                      
Impact of Recently Issued Accounting Standards [Abstract]                      
Revenue                 17,000 27,000  
Deferred revenue 39,000               39,000    
Accumulated deficit $ 39,000               39,000    
Income tax expense                 $ 0 $ 0