Fair Value Measurements - Additional Information (Detail) - USD ($) $ in Millions	1 Months Ended	3 Months Ended	12 Months Ended
Mar. 31, 2021	Mar. 31, 2022	Dec. 31, 2021
Fair value transfers into Level 3		$ 0.0	$ 0.0
Fair value transfers out of Level 3		0.0	$ 0.0
Assets or liabilities measured at fair value on a non-recurring basis.		$ 0.0
CIP related assets	$ 4.0
Lonza Manufacturing Agreement [Member]
Commitment to procure equipment and construction	$ 23.0

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

- Definition

Commitment to procure equipment and construction

+ References

No definition available.

+ Details

Name:	alt_CommitmentToProcureEquipmentAndConstruction
Namespace Prefix:	alt_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of fair value of construction in progress.

+ References

No definition available.

+ Details

Name:	alt_FairValueOfConstructionInProgress
Namespace Prefix:	alt_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of transfers of financial instrument classified as a derivative asset (liability) after deduction of derivative liability (asset) into level 3 of the fair value hierarchy.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=123874694&loc=d3e19279-110258

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(3)
-URI http://asc.fasb.org/extlink&oid=123874694&loc=d3e19207-110258

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of transfers of financial instrument classified as derivative asset (liability) after deduction of derivative liability (asset) out of level 3 of the fair value hierarchy.

+ References

+ Details

Name:	us-gaap_FairValueNetDerivativeAssetLiabilityMeasuredOnRecurringBasisUnobservableInputsReconciliationTransfersOutOfLevel3
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	srt_CounterpartyNameAxis=alt_LonzaManufacturingAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: