Collaboration Agreements and Revenues - Transaction Price Related to Consideration Received and Accounts Receivable Allocated to Performance Obligations along with Associated Deferred Revenue (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Deferred Revenue	$ 0	$ 54,790
Astellas Agreement [Member] \| Japan [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Cumulative Revenue	101,567
Deferred Revenue	164
Total Consideration	101,731
Astellas Agreement [Member] \| Japan [Member] \| Development Revenue [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Cumulative Revenue	15,543
Deferred Revenue	164
Total Consideration	15,707
Astellas Agreement [Member] \| Japan [Member] \| License Revenue [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Cumulative Revenue	86,024
Deferred Revenue	0
Total Consideration	86,024
Astellas Agreement [Member] \| Europe [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Cumulative Revenue	732,785
Deferred Revenue	2,096
Total Consideration	734,881
Astellas Agreement [Member] \| Europe [Member] \| Development Revenue [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Cumulative Revenue	244,834
Deferred Revenue	2,096
Total Consideration	246,930
Astellas Agreement [Member] \| Europe [Member] \| License Revenue [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Cumulative Revenue	487,951
Deferred Revenue	0
Total Consideration	487,951
AstraZeneca Agreements [Member] \| U.S./RoW and China [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Cumulative Revenue	880,026
Deferred Revenue	143,606
Total Consideration	1,023,632
AstraZeneca Agreements [Member] \| U.S./RoW and China [Member] \| License Revenue [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Cumulative Revenue	341,844
Deferred Revenue	0
Total Consideration	341,844
AstraZeneca Agreements [Member] \| U.S./RoW and China [Member] \| Co-development, information sharing & committee services [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Cumulative Revenue	536,792
Deferred Revenue	2,435
Total Consideration	539,227
AstraZeneca Agreements [Member] \| U.S./RoW and China [Member] \| China performance obligation [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Cumulative Revenue	1,390
Deferred Revenue	141,171
Total Consideration	$ 142,561

Collaboration Agreements and Revenues - Transaction Price Related to Consideration Received and Accounts Receivable Allocated to Performance Obligations along with Associated Deferred Revenue (Detail) - USD ($)
$ in Thousands