Note 4 - Fair Value Measurements (Details Textual)	3 Months Ended	12 Months Ended
Sep. 30, 2017	Jun. 30, 2017
Available-for-Sale Securities, Maturity Period	1 year	1 year

Represents the period over which available-for-sale securities mature.

No definition available.

Name:	avir_AvailableForSaleSecuritiesMaturityPeriod
Namespace Prefix:	avir_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration