Collaboration Agreements - Components of Collaboration Revenue (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2019	Jun. 30, 2018
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total collaboration revenue	$ 406	$ 21,318	$ 5,934	$ 14,034	$ 5,416	$ 3,603	$ 4,852	$ 10,315	$ 41,692	$ 24,186
Collaboration Agreement [Member] | BIOGEN [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total collaboration revenue									41,128	24,057
Collaboration Agreement [Member] | BIOGEN [Member] | Licenses and Related Services
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total collaboration revenue									27,000	18,529
Collaboration Agreement [Member] | BIOGEN [Member] | Development Services
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total collaboration revenue									2,736	3,028
Collaboration Agreement [Member] | BIOGEN [Member] | Milestone Revenue
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total collaboration revenue									$ 11,392	$ 2,500