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Fair Value Measurements - Additional Information (Detail)	9 Months Ended
	Sep. 30, 2019
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Maximum maturity period of cash and cash equivalents	90 days
Maximum maturity period of short-term investments	1 year
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

Name:	achv_MaximumMaturityPeriodOfCashAndCashEquivalents
Namespace Prefix:	achv_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Maximum maturity period of short term investments.

+ References

No definition available.

+ Details

Name:	achv_MaximumMaturityPeriodOfShortTermInvestments
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: