Collaborative Arrangements - Merck KGaA (Details) - Merck KGaA $ in Millions	1 Months Ended	12 Months Ended
	Jan. 31, 2017 developmentProgram clinical-stage_program	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Right to license, number of development programs | developmentProgram	4
Right to license, number of clinical stage programs | clinical-stage_program	2
Number of pre-clinical stage programs | developmentProgram	2
Collaborative arrangement development and regulatory potential milestone payments maximum | $			$ 230.0
Up-front payment | $		$ 65.0