Other Income - Additional Information (Detail) $ in Thousands	1 Months Ended
Aug. 31, 2015 USD ($)
Other Income
Other Income [Line Items]
Disgorgement of short swing profits under Section 16(b) of the Securities Exchange Act	$ 8,944

Disgorgement of short swing profits under section 16 B of the Securities Exchange Act

No definition available.

Name:	achn_DisgorgementOfShortSwingProfitsUnderSection16bOfTheSecuritiesExchangeAct
Namespace Prefix:	achn_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration