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Financial Instruments and Risk - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2017
Schedule Of Cost Method Investments [Line Items]
Average days to maturity of Company's portfolio of cash equivalents and marketable securities	0 days
Maximum [Member]
Schedule Of Cost Method Investments [Line Items]
Maturity period of securities	1 year

X

- Definition

Average period to maturity for portfolio of cash and marketable securities.

+ References

No definition available.

+ Details

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

X

- Definition

Maturity period of securities.

+ 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_ScheduleOfCostMethodInvestmentsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Details

Name:	us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: