Collaborative Arrangements - GSK Platform Technology Transfer, Collaboration and License Agreement (Details) - USD ($)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019	Jul. 31, 2014
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Revenue recognized	$ 14,967,000	$ 12,319,000	$ 29,637,000	$ 27,901,000
GSK [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Revenue recognized	$ 0	$ 0	$ 0	$ 0	$ 2,000,000.0
GSK [Member] | Minimum [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Contingent receivable						$ 5,750,000
GSK [Member] | Maximum [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Contingent receivable						$ 38,500,000