Operating Leases (Details) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($) Option	Jun. 30, 2018 USD ($)
Operating Leases [Abstract]
Right-of-use operating lease assets	[1]	$ 12,900		$ 12,900
Operating lease liabilties	[2]	$ 17,677		$ 17,677
Weighted average remaining lease term		8 years 8 months 12 days		8 years 8 months 12 days
Weighted average discount rate		7.60%		7.60%
Current portion of operating lease liabilties		$ 1,900		$ 1,900
Operating lease cash payments				2,000
Annual Future Minimum Payments Under Operating Leases [Abstract]
Remainder of 2019		1,548		1,548
2020		3,124		3,124
2021		2,855		2,855
2022		2,607		2,607
2023		2,505		2,505
Thereafter		11,861		11,861
Total minimum lease payments		24,500		24,500
Imputed interest		(6,823)		(6,823)
Total lease liabilities	[2]	17,677		17,677
Rent expense		$ 1,000	$ 900	$ 1,900	$ 900
Office and Laboratory Space Adjacent to Manufacturing Facility [Member]
Operating Leases [Abstract]
Number of options to extend lease | Option				2
Term of lease extension		5 years		5 years
Office Space Subleased to Akcea [Member]
Operating Leases [Abstract]
Number of options to extend lease | Option				1
Term of lease extension		5 years		5 years
Akcea [Member] | Office Space for Corporate Headquarters [Member]
Operating Leases [Abstract]
Term of lease extension		5 years		5 years
Term of lease		123 months		123 months
Period of free rent under lease				3 months
Tenant improvement allowance		$ 3,800		$ 3,800
Initial amount of letter of credit		2,400		2,400
Letter of credit on third anniversary of rent commencement date		1,800		1,800
Letter of credit on fifth anniversary of rent commencement date		$ 1,200		$ 1,200

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

[2] Current portion of $1.9 million was included in current portion of long-term obligations on our condensed consolidated balance sheet, with the difference included in long-term obligations.