Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Summary of Significant Accounting Policies
Discrete tax benefit			$ 0.7
Income tax expense(benefit) other than discrete income tax expense (benefit)	$ 0.0	$ 0.0	$ 0.0	$ 0.0

The amount of tax advantage recognized separately from the annual effective tax rate during interim financial reporting periods. It typically arises from specific events or transactions that occur within a particular period and are not part of the ordinary income expected for the fiscal year.

No definition available.

Name:	alt_DiscreteIncomeTaxExpenseBenefit
Namespace Prefix:	alt_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration