Collaborative Arrangements - Strategic Collaboration and License Agreement (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Mar. 31, 2020	Dec. 31, 2019
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Contract with customer, liability	$ 5,045		$ 5,045			$ 2,044
Revenue recognized	14,967	$ 12,319	29,637	$ 27,901
Takeda [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Contract with customer, liability	2,200		2,200
Revenue recognized	$ 5,700		$ 8,000
Takeda [Member] | Up-front Payment [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Contract with customer, liability					$ 8,500
Takeda [Member] | Research and Development Reimbursement [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Contingent receivable					22,300
Takeda [Member] | Milestone Payment Per Target Gene [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Contingent receivable					$ 100,000