Fair Value Measurements - Additional Information (Details) - USD ($)	3 Months Ended	6 Months Ended	12 Months Ended
Jun. 30, 2020	Jun. 30, 2020	Dec. 31, 2019
Fair Value Disclosures [Abstract]
Change in valuation methods or transfers	$ 0	$ 0	$ 0
Transfers within fair value hierarchy	$ 0	$ 0	$ 0

Fair value measurement amount of changes in valuation method or transfer.

No definition available.

Name:	alna_FairValueMeasurementAmountOfChangesInValuationMethodOrTransfer
Namespace Prefix:	alna_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration