Summary of Revenue from Collaboration and License Agreements (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration and license revenue	$ 3,828	$ 9,322	$ 12,743	$ 21,811
Bristol-Myers Squibb Company ("BMS") and Pfizer Inc. ("Pfizer")
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration and license revenue	1,038	1,798	3,824	4,785
Daiichi Sankyo, Inc ("Daiichi")
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration and license revenue	2,034	3,810	6,227	8,491
Bayer Pharma, AG ('Bayer") and Janssen Pharmaceuticals, Inc. ("Janssen")
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration and license revenue	544	3,316	1,880	7,431
Bayer Pharma AG
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration and license revenue	$ 212	$ 398	$ 812	1,052
Other
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration and license revenue				$ 52