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Stockholders' Equity - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands | 1 Months Ended | 12 Months Ended
--- | --- | ---
Aug. 31, 2020 | Jun. 30, 2020 | Sep. 30, 2019 | Jun. 30, 2019 | Jun. 30, 2018 | Jun. 30, 2017 | Jun. 30, 2016 | Jun. 30, 2015 | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2010
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Issuance of common stock in connection with asset acquisition |  |  |  |  |  |  |  |  | $ 44,280 |  |  |
Net proceeds from sale of common stock |  |  | $ 271,500 |  |  |  |  |  | $ 80,996 | $ 371,847 | $ 306,647 |
Issuance of common stock in public offering, net of issuance costs (in shares) |  |  | 7,187,500 |  |  |  |  |  |  |  |  |
Options vesting period |  |  |  |  |  |  |  |  | 4 years |  |  |
Closing market price of Common stock |  |  |  |  |  |  |  |  | $ 53.46 |  |  |
Aggregate intrinsic value of stock options exercised |  |  |  |  |  |  |  |  | $ 55,500 | $ 56,600 | $ 11,200 |
Cash received from options exercised |  |  |  |  |  |  |  |  | $ 73,800 |  |  |
Weighted average per share fair value of options granted |  |  |  |  |  |  |  |  | $ 24.16 | $ 15.97 | $ 12.14 |
Unrecognized compensation costs |  |  |  |  |  |  |  |  | $ 99,100 |  |  |
Weighted average period cost expected to be recognized |  |  |  |  |  |  |  |  | 2 years 6 months |  |  |
Terms of award |  |  |  |  |  |  |  |  | PSUs for which the number of shares issuable at the end of performance period can reach up to 200% of the shares approved in the award based on the achievement of certain pre-defined Acadia-specific performance criteria and continued employment through the vesting period |  |  |
Issuance of common stock pursuant to employee stock purchase plan |  |  |  |  |  |  |  |  | $ 7,162 | $ 4,411 | $ 3,682 |
Stock Options | Equity Incentive Plan 2010 |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Exercise price of Options as percentage of fair market value |  |  |  |  |  |  |  |  |  |  |  | 100.00%
Options maximum expiration term |  |  |  |  |  |  |  |  |  |  |  | 10 years
Options vesting period |  |  |  |  |  |  |  |  |  |  |  | 4 years
Number of common stock authorized for issuance |  |  |  |  |  |  |  |  | 28,426,504 |  |  |
Common stock available for grants |  |  |  |  |  |  |  |  | 9,777,263 |  |  |
Common stock reserved for issuance under stock purchase plan |  |  |  | 8,300,000 | 6,700,000 | 5,500,000 | 3,000,000 | 5,000,000 |  |  |  |
Stock Options | Equity Incentive Plan 2004 |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Exercise price of Options as percentage of fair market value |  |  |  |  |  |  |  |  | 100.00% |  |  |
Options vesting period |  |  |  |  |  |  |  |  | 4 years |  |  |
Number of periods additional number of shares could be added to shares authorized for issuance |  |  |  |  |  |  |  |  | 10 years |  |  |
PSU |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Unrecognized compensation costs |  |  |  |  |  |  |  |  | $ 2,600 |  |  |
Weighted average period cost expected to be recognized |  |  |  |  |  |  |  |  | 1 year 6 months |  |  |
Granted |  |  |  |  |  |  |  |  | 255,878 | 481,036 |  |
Aggregate Intrinsic Value, Outstanding |  |  |  |  |  |  |  |  | $ 37,400 |  |  |
Outstanding shares |  |  |  |  |  |  |  |  | 700,156 | 448,632 |  |
Restricted Stock Units |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Unrecognized compensation costs |  |  |  |  |  |  |  |  | $ 41,700 |  |  |
Weighted average period cost expected to be recognized |  |  |  |  |  |  |  |  | 2 years 8 months 12 days |  |  |
Employee Stock Purchase Plan |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Number of common stock authorized for issuance |  |  |  |  |  |  |  |  | 5,525,000 |  |  |
Common stock reserved for issuance under stock purchase plan |  | 3,000,000 |  | 600,000 |  |  | 400,000 |  |  |  |  |
Common stock remained available for issuance pursuant to the Purchase Plan |  |  |  |  |  |  |  |  | 3,106,995 |  |  |
Eligible employees percentage of earnings withheld to purchase shares under purchase plan |  |  |  |  |  |  |  |  | 15.00% |  |  |
Purchase price of common stock as percentage of market value |  |  |  |  |  |  |  |  | 85.00% |  |  |
Issuance of common stock pursuant to employee stock purchase plan |  |  |  |  |  |  |  |  | 377,963 | 269,037 | 233,720 |
Average per share price of stock issued under purchase plan |  |  |  |  |  |  |  |  | $ 18.94 | $ 16.41 | $ 15.75 |
Weighted average per share fair value of purchase rights granted |  |  |  |  |  |  |  |  | $ 22.47 | $ 14.24 | $ 8.25 |
Issuance of common stock pursuant to employee stock purchase plan |  |  |  |  |  |  |  |  | $ 7,200 | $ 4,400 | $ 3,700 |
Option To Purchase Additional Shares |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Issuance of common stock in public offering, net of issuance costs (in shares) |  |  | 937,500 |  |  |  |  |  |  |  |  |
Cersci Therapeutics |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Issuance of common stock in connection with asset acquisition, Shares | 1,200,000 |  |  |  |  |  |  |  |  |  |  |
Issuance of common stock in connection with asset acquisition | $ 44,300 |  |  |  |  |  |  |  |  |  |  |

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