Long-Term Obligations and Commitments, Operating Leases (Details) $ in Thousands		12 Months Ended
		Dec. 31, 2020 USD ($) Option	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Operating Leases [Abstract]
Right-of-use operating lease assets	[1]	$ 13,100
Operating Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List]		us-gaap:OtherAssetsNoncurrent
Operating lease liabilities	[2]	$ 17,310
Weighted average remaining lease term		7 years 2 months 12 days
Weighted average discount rate		7.00%
Current portion of operating lease liabilities		$ 2,000
Operating Lease, Liability, Current, Statement of Financial Position [Extensible List]		us-gaap:LongTermDebtAndCapitalLeaseObligationsCurrent
Operating Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]		ions:LongTermObligationsNoncurrent
Operating lease cash payments		$ 3,800	$ 3,900	$ 1,700
Annual Future Payments for Operating Lease Liabilities [Abstract]
2021		3,193
2022		2,968
2023		2,707
2024		2,583
2025		2,442
Thereafter		7,038
Total minimum lease payments		20,931
Imputed interest		(3,621)
Total operating lease liabilities	[2]	17,310
Rent expense		$ 3,700	$ 3,600	$ 2,600
Carlsbad Facility [Member]
Operating Leases [Abstract]
Number of options to extend lease | Option		1
Term of lease extension		5 years
Carlsbad Office Spaces [Member]
Operating Leases [Abstract]
Number of options to extend lease | Option		1
Term of lease extension		5 years
Akcea [Member] | Boston Office Space [Member]
Operating Leases [Abstract]
Term of lease extension		5 years
Term of lease		123 months
Period of free rent under lease		3 months
Tenant improvement allowance		$ 3,800
Initial amount of letter of credit		2,400
Letter of credit on third anniversary of rent commencement date		1,800
Letter of credit on fifth anniversary of rent commencement date		$ 1,200

[1] Included in deposits and other assets on our consolidated balance sheet.

[2] Current portion of $2.0 million was included in current portion of long-term obligations on our consolidated balance sheet, with the difference included in long-term obligations.