Development, Commercialization and License Agreement and Services Agreement with Ionis (Details) $ in Thousands	3 Months Ended			6 Months Ended
	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) Patient Payment	Jun. 30, 2016 USD ($)
Operating Expenses Generated by Transactions with Ionis [Abstract]
Payable balance to Ionis at beginning of the period		$ 24,355		$ 24,355
Total amount payable to Ionis at period end	$ 11,244			$ 11,244
Ionis [Member] | Development, Commercialization and License Agreement [Member]
Development, Commercialization and License Agreement with Ionis [Abstract]
Royalty percentage paid on sales of lipid drug				20.00%
First annual sales threshold				$ 500,000
Second annual sales threshold				1,000,000
Third annual sales threshold				2,000,000
Sales milestone payment				$ 50,000
Number of quarters in which equal payments are made | Payment				12
Ionis [Member] | Development, Commercialization and License Agreement [Member] | Lipid Drug for Rare Disease Indication [Member] | Maximum [Member]
Development, Commercialization and License Agreement with Ionis [Abstract]
Number of patients worldwide | Patient				500,000
Number of patients for Phase 3 program | Patient				1,000
Number of years of treatment				2 years
Ionis [Member] | Development, Commercialization and License Agreement [Member] | Lipid Drug for a Broad Disease Patient Population [Member] | Minimum [Member]
Development, Commercialization and License Agreement with Ionis [Abstract]
Number of patients worldwide | Patient				500,000
Number of patients for Phase 3 program | Patient				1,000
Number of years of treatment				2 years
Ionis [Member] | Services Agreement [Member]
Services Agreement with Ionis [Abstract]
Payment term after receipt of invoice				30 days
Term of extension for services agreement				6 months
Operating Expenses Generated by Transactions with Ionis [Abstract]
Services performed by Ionis	2,998		$ 2,059	$ 5,955	$ 4,112
Active pharmaceutical ingredient manufactured by Ionis	2,930		0	6,013	0
Sublicensing expenses	0		0	48,394	0
Out-of-pocket expenses paid by Ionis	5,316		8,123	17,189	17,974
Total expenses generated by transactions with Ionis	11,244		10,182	77,551	22,086
Payable balance to Ionis at beginning of the period	15,000	24,355	21,102	24,355	9,198
Less: total amounts paid to Ionis during the period	(15,000)		0	(57,268)	0
Less: non-cash sublicensing expenses	0		0	(33,394)	0
Total amount payable to Ionis at period end	$ 11,244	$ 15,000	$ 31,284	$ 11,244	$ 31,284