Collaboration Agreements - Summary of Performance Obligations and Transaction Price (Detail) - Collaboration Agreement [Member] - BIOGEN [Member] - Accounting Standards Update 2014-09 [Member] $ in Thousands	Jun. 30, 2019 USD ($)
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Allocated Transaction Price	$ 112,330
XLRS [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Allocated Transaction Price	52,060
XLRP [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Allocated Transaction Price	43,570
Discovery Programs [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Allocated Transaction Price	$ 16,700