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	Jun. 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	101,482
Deferred Revenue	183
Total Consideration	101,665
Astellas Agreement [Member] \| Japan [Member] \| Development Revenue [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Cumulative Revenue	15,458
Deferred Revenue	183
Total Consideration	15,641
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	728,135
Deferred Revenue	2,338
Total Consideration	730,473
Astellas Agreement [Member] \| Europe [Member] \| Development Revenue [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Cumulative Revenue	240,184
Deferred Revenue	2,338
Total Consideration	242,522
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	864,100
Deferred Revenue	145,520
Total Consideration	1,009,620
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	521,572
Deferred Revenue	4,547
Total Consideration	526,119
AstraZeneca Agreements [Member] \| U.S./RoW and China [Member] \| China performance obligation [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Cumulative Revenue	684
Deferred Revenue	140,973
Total Consideration	$ 141,657

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