Share based compensation maximum annual increase in number of shares that may be issued under the plan.

No definition available.

Name:	atha_ShareBasedCompensationMaximumAnnualIncreaseInNumberOfSharesThatMayBeIssuedUnderPlan
Namespace Prefix:	atha_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Share based compensation percentage of number shares of common stock issued and outstanding on immediate proceeding.

+ References

No definition available.

+ Details

Name:	atha_ShareBasedCompensationPercentageOfNumberSharesOfCommonStockIssuedAndOutstandingOnImmediateProceeding
Namespace Prefix:	atha_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_ClassOfStockLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Aggregate number of common shares reserved for future issuance.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.29)
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

Name:	us-gaap_CommonStockCapitalSharesReservedForFutureIssuance
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	us-gaap_PlanNameAxis=atha_TwoThousandAndTwentyEquityIncentivePlanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PlanNameAxis=atha_TowThousandAndTwentyEmployeeStockPurchasePlanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Common Stock - Additional Information (Details) - shares	6 Months Ended
	Jun. 30, 2022	Jan. 01, 2022	Dec. 31, 2021	Jan. 01, 2021
Class Of Stock [Line Items]
Reserved shares of common stock for future issuance	9,500,018		7,695,922
2020 Equity Incentive Plan
Class Of Stock [Line Items]
Share based compensation maximum annual increase in number of shares that may be issued under the plan	3,230,000
Share based compensation percentage of number shares of common stock issued and outstanding on immediate proceeding	5.00%
Reserved shares of common stock for future issuance		1,868,953		1,624,259
2020 Employee Stock Purchase Plan
Class Of Stock [Line Items]
Share based compensation maximum annual increase in number of shares that may be issued under the plan	646,000
Share based compensation percentage of number shares of common stock issued and outstanding on immediate proceeding	1.00%
Reserved shares of common stock for future issuance		373,790		324,851