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Equity - Common Stock - Additional Information (Detail) $ / shares in Units, $ in Millions |  |  |  | 9 Months Ended |  |
--- | --- | --- | --- | --- | --- | ---
Mar. 16, 2020 £ / shares shares | Jul. 29, 2019 USD ($) | Jul. 18, 2019 $ / shares shares | Sep. 30, 2020 £ / shares shares | Sep. 30, 2019 shares | Dec. 31, 2019 £ / shares | Mar. 05, 2015 £ / shares
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |
Common stock, par value | £ / shares |  |  |  | £ 0.50 |  | £ 0.50 | £ 0.50
2011 Plan |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |
Number of shares remaining under the plan |  |  |  | 2,634,440 |  |  |
Amarin Corporation Plc 2017 Employee Stock Purchase Plan | Ordinary Shares |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |
Issuance of common stock, net of transaction costs (shares) |  |  |  | 123,608 | 47,358 |  |
American Depositary Shares |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |
Issuance of common stock, net of transaction costs (shares) |  |  | 22,222,223 |  |  |  |
American Depository Shares Ratio |  |  | 1 |  |  |  |
Sale of stock price per share | $ / shares |  |  | $ 18.00 |  |  |  |
American Depositary Shares | Over-Allotment Option |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |
Sale of stock price per share | $ / shares |  |  | $ 18.00 |  |  |  |
Issuance of stock (shares), exercised |  |  | 3,333,333 |  |  |  |
Gross proceeds from public offering including exercised option | $ |  | $ 460.0 |  |  |  |  |
Net proceeds from public offering including exercised option | $ |  | $ 440.1 |  |  |  |  |
American Depositary Shares | Stock Incentive Plan 2020 |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |
Expiration date of stock incentive plan | 2030-07 |  |  |  |  |  |
American Depositary Shares | Stock Incentive Plan 2020 | Ordinary Shares |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |
Common stock, par value | £ / shares | £ 0.50 |  |  |  |  |  |
Issuance of common stock, net of transaction costs (shares) | 20,000,000 |  |  |  |  |  |

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