Collaborative Arrangements - Merck KGaA (Details) - Merck KGaA $ in Millions	1 Months Ended		12 Months Ended
	Dec. 31, 2018 USD ($)	Jan. 31, 2017 pre-clinical_stage_program clinical-stage_program development_program	Dec. 31, 2017 USD ($)
Collaborative Arrangement and Arrangement Other than Collaborative [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
Collaborative arrangement development and regulatory potential milestone payments maximum			$ 230.0
Up-front payment	$ 65.0