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Equity - Preferred Stock - Additional Information (Detail) |  | 3 Months Ended | 9 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | ---
Mar. 05, 2015 USD ($) shares | Mar. 31, 2015 USD ($) | Sep. 30, 2021 £ / shares shares | Dec. 31, 2020 £ / shares shares | Dec. 31, 2018 shares | Dec. 31, 2015 shares | Mar. 05, 2015 £ / shares | Mar. 05, 2015 $ / shares
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |
Preferred stock, par value | £ / shares |  |  |  |  |  |  | £ 0.05 |
Estimated offering expense | $ |  | $ 700,000 |  |  |  |  |  |
Common stock, par value | £ / shares |  |  | £ 0.50 | £ 0.50 |  |  | £ 0.50 |
Series A Preferred Stock |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |
American depositary shares conversion rate to preference shares |  |  | American Depositary Shares, or ADSs, each representing one share of Amarin’s Series A Convertible Preference Shares |  |  |  |  |
Ordinary Shares |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |
Convertible senior notes, total ADS into which the debt is exchangeable |  |  |  | 28,931,746 | 3,886,718 | 6,283,333 |  |
Preferred stock, equivalent ordinary shares upon future consolidation outstanding |  |  | 0 | 0 |  |  |  |
American Depositary Shares | Series A Preferred Stock |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |
Issuance of stock (shares) | 391,017,970 |  |  |  |  |  |  |
Stock issued under private placement, value | $ | $ 58,600,000 |  |  |  |  |  |  |
Preferred Shares Basis |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |
Share conversion price | $ / shares |  |  |  |  |  |  |  | $ 0.15
Ordinary Shares Basis |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |
Share conversion price | $ / shares |  |  |  |  |  |  |  | $ 1.50

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