Collaboration Agreements - Merck KGaA - Accounting Analysis (Details)	3 Months Ended								12 Months Ended
	Dec. 31, 2020 USD ($) target	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2020 USD ($) target	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($) target	Jan. 01, 2018 USD ($)	Dec. 31, 2017 target
Collaboration agreements
Collaboration revenue	$ 10,000	$ 11,000	$ 796,000	$ 11,000	$ 42,000	$ 844,000	$ 202,000	$ 41,035,000	$ 828,000	$ 42,123,000	$ 10,594,000
Deferred revenue	3,987,000				4,815,000				3,987,000	4,815,000	$ 46,196,000
Merck KGaA Agreement and Merck KGaA supply agreement
Collaboration agreements
Accounts receivable	0				0				0	0
Deferred revenue	$ 3,987,000				4,815,000				$ 3,987,000	4,815,000
Merck KGaA
Collaboration agreements
Number of targets designated | target	6								6		6		6
Amount of milestone payments, which had not been received, included in the estimated transaction price												$ 0
Transaction price	$ 21,325,000				$ 21,500,000				$ 21,325,000	21,500,000
Number of service obligations completed of designated targets | target	4								4
Amount of research and development fees expected to be received	$ 6,325,000								$ 6,325,000
Decrease in fees expected to be received for research and development activities									175,000
Collaboration revenue									828,000	853,000	$ 2,444,000
Merck KGaA | License and corresponding research and development services, first and second targets
Collaboration agreements
Transaction price	3,941,000								3,941,000
Merck KGaA | License and corresponding research and development services, third and sixth target
Collaboration agreements
Transaction price	3,439,000								3,439,000
Merck KGaA | License and corresponding research and development services, fourth target
Collaboration agreements
Transaction price	3,152,000								3,152,000
Merck KGaA | License and corresponding research and development services, fifth target
Collaboration agreements
Transaction price	2,746,000								2,746,000
Merck KGaA | Rights to future technological improvements
Collaboration agreements
Transaction price	425,000								425,000
Merck KGaA | Joint research committee services
Collaboration agreements
Transaction price	$ 242,000								242,000
Merck KGaA supply agreement
Collaboration agreements
Collaboration revenue									$ 0	$ 1,280,000	$ 0
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-01-01 | Merck KGaA | Rights to future technological improvements
Collaboration agreements
Expected recognition period	10 years								10 years
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-01-01 | Merck KGaA | Joint research committee services
Collaboration agreements
Expected recognition period	5 years								5 years