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Fair Value Measurements - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2018 USD ($)
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Warrant liability	$ 0
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

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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

Maximum maturity period of short term investments.

+ References

No definition available.

+ Details

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Namespace Prefix:	achv_
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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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X

- Definition

Fair value of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

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- Details

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