Collaboration Agreements - MERCK Revenue and Contra-Revenue Recognized (Details) - Collaboration revenue - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Contra-revenue under the May 2019 Amendment
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Total collaboration revenue	$ (7,088)	$ (5,265)	$ 0
Collaboration revenue under the 2015 Merck Agreement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Total collaboration revenue	62	492	24,582
Separate agreements with Merck
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Total collaboration revenue	$ (7,026)	$ (4,773)	$ 24,582