Collaboration Agreements and Related Revenue Matters - Components of Collaboration Revenue (Detail) - USD ($) $ in Thousands	3 Months Ended					12 Months Ended
	Dec. 31, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2020	Jun. 30, 2019
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total collaboration revenue	$ 2,453	$ 406	$ 21,318	$ 5,934	$ 14,034	$ 2,453	$ 41,692
Collaboration Agreement [Member] | BIOGEN [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total collaboration revenue						$ 0	41,128
Collaboration Agreement [Member] | BIOGEN [Member] | Licenses and Related Services [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total collaboration revenue							27,000
Collaboration Agreement [Member] | BIOGEN [Member] | Development Services [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total collaboration revenue							2,736
Collaboration Agreement [Member] | BIOGEN [Member] | Milestone Revenue [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total collaboration revenue							$ 11,392