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Investments - Additional Information (Detail) - USD ($) $ in Millions | Jun. 30, 2018 | Dec. 31, 2017
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Investments Debt And Equity Securities [Abstract] |  |
Available-for-sale, investment securities held with maturity dates more than one year and less than two years | $ 44.2 | $ 48.7

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- Definition Available for sale securities debt maturities rolling year one through two. + References No definition available. + Details
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Namespace Prefix: | acad_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration