Fair Value Measurements - Additional Information (Detail) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Fair Value Measurements
Assets or liabilities measured at fair value on a non-recurring basis.	$ 0	$ 0

Assets or liabilities that were measured at fair value on a non-recurring basis.

No definition available.

Name:	alt_AssetsOrLiabilitiesMeasuredAtFairValueOnNonRecurringBasis
Namespace Prefix:	alt_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant