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Fair Value - Additional Information (Details) - Recurring Basis | 12 Months Ended
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Dec. 31, 2023
Minimum |
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items] |
Long-term investments effective maturity date | 1 year
Maximum |
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items] |
Short-term investments effective maturity date | 1 year
Long-term investments effective maturity date | 2 years

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- Definition Long-term investments maturity term. + References No definition available. + Details
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Namespace Prefix: | atha_
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Balance Type: | na
Period Type: | duration

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- Definition Short-term investments maturity term. + References No definition available. + Details
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Namespace Prefix: | atha_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- 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
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Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
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Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
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Period Type: |