Collaborative Arrangements - Merck KGaA (Details)		3 Months Ended								12 Months Ended
	Jan. 10, 2017 pre-clinical_stage_program clinical-stage_program development_program	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaborative revenues		$ 29,061,000	$ 26,292,000	$ 27,286,000	$ 232,545,000	$ 937,000	$ 259,000	$ 675,000	$ 74,000	$ 315,184,000	$ 1,945,000	$ 8,053,000
Merck KGaA
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					230,000,000.0					230,000,000.0
Proceeds from collaborators					193,600,000
Collaborative revenues, related to upfront payment upon delivery of license and to research and development transition activities										246,600,000
Collaborative revenues										$ 16,600,000
Time period of notice required to terminate	90 days
Time period of notice required to terminate after product has received marketing approval	180 days
German tax authority | Merck KGaA
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaborative revenues, tax withholding					$ 36,400,000