Collaborative Arrangements - Merck KGaA (Details) - Merck KGaA	1 Months Ended		3 Months Ended
	Dec. 31, 2018 USD ($)	Jan. 31, 2017 USD ($) pre-clinical_stage_program clinical-stage_program development_program	Mar. 31, 2017 USD ($)
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Right to license, number of development programs | development_program		4
Right to license, number of clinical stage programs | clinical-stage_program		2
Number of pre-clinical stage programs | pre-clinical_stage_program		2
Up-front payment	$ 65,000,000		$ 230,000,000.0
Collaborative arrangement development and regulatory potential milestone payments maximum		$ 230,000,000