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Fair Value Measurements - Additional Information (Detail) | 9 Months Ended
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Sep. 30, 2020
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |
Maximum maturity period of cash and cash equivalents | 90 days
Maximum [Member] |
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |
Maturity period of securities | 1 year

X
- Definition Maturity period of securities. + References No definition available. + Details
Name: | achv_MaturityPeriodOfSecurities
Namespace Prefix: | achv_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Maximum maturity period of cash and cash equivalents. + References No definition available. + Details
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Namespace Prefix: | achv_
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
Name: | us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | srt_RangeAxis=srt_MaximumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |