Liability Related to Sale of Future Royalties, net - Activity within Liability Account (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Liability For Sale Of Future Royalties [Roll Forward]
Liability related to sale of future royalties - beginning balance	$ 144,111
Non-cash interest expense on liability related to sale of future royalties	8,425	$ 6,817
Liability related to sale of future royalties - ending balance	151,454
RPI Funding Agreement
Liability For Sale Of Future Royalties [Roll Forward]
Liability related to sale of future royalties - beginning balance	144,111	117,515
Royalty revenues payable to RPI	(24)	0
Non-cash interest expense on liability related to sale of future royalties	8,425	6,817
Liability related to sale of future royalties - ending balance	$ 152,512	$ 124,332