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Investments - Additional Information (Detail) $ in Millions	Sep. 30, 2018 USD ($) Security	Dec. 31, 2017 USD ($) Security
Investments Debt And Equity Securities [Abstract]
Available-for-sale, investment securities held with maturity dates more than one year and less than two years | $	$ 35.1	$ 48.7
Number of securities in unrealized loss position | Security	37	54

X

- 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

X

- Definition

Number of investments in debt securities measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (b)(3)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

X

- References

No definition available.

+ Details

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