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Equity - Preferred Stock - Additional Information (Detail) |  |  |  |  |  | 1 Months Ended | 3 Months Ended | 12 Months Ended |  |
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Jul. 18, 2019 shares | Feb. 01, 2018 shares | Jul. 10, 2015 USD ($) $ / shares shares | Mar. 30, 2015 USD ($) | Mar. 05, 2015 USD ($) shares | Mar. 31, 2020 | Mar. 31, 2015 USD ($) | Dec. 31, 2020 £ / shares shares | Dec. 31, 2019 USD ($) | Dec. 31, 2018 USD ($) shares | Dec. 31, 2015 shares | Dec. 31, 2019 £ / shares shares | Mar. 05, 2015 £ / shares shares | Mar. 05, 2015 $ / shares shares
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Preferred stock, shares outstanding |  |  |  |  |  |  |  | 0 |  |  |  | 289,317,460 |  |
Preferred stock, par value | £ / shares |  |  |  |  |  |  |  | £ 0.05 |  |  |  | £ 0.05 | £ 0.05 |
Private placement, closing date |  |  |  |  | Mar. 30, 2015 |  |  |  |  |  |  |  |  |
Stock issued under private placement, value | $ |  |  |  |  |  |  |  |  | $ 440,108,000 | $ 264,840,000 |  |  |  |
Estimated offering expense | $ |  |  |  |  |  |  | $ 700,000 |  |  |  |  |  |  |
Common stock, par value | £ / shares |  |  |  |  |  |  |  | £ 0.50 |  |  |  | £ 0.50 | £ 0.50 |
Number of shares issued for each preference shares |  |  |  |  |  |  |  |  |  |  |  |  | 0.1 | 0.1
Preferred stock, equivalent ordinary shares upon future consolidation outstanding |  |  |  |  |  |  |  |  |  |  |  | 28,931,746 |  |
Series A Preferred Stock |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
American depositary shares conversion rate to preference shares |  |  |  |  |  |  |  | American Depositary Shares, each representing one (1) share of Amarin’s Series A Convertible Preference Shares |  |  |  |  |  |
Series A Preferred Stock | Minimum |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Beneficial ownership limitation |  |  |  |  | 4.99% | 9.90% |  |  |  |  |  |  |  |
Series A Preferred Stock | Maximum |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Beneficial ownership limitation |  |  |  |  | 19.90% |  |  |  |  |  |  |  |  |
Series A Preferred Stock | Maximum | Second Private Placement |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Preferred stock, equivalent ordinary shares upon future consolidation outstanding |  |  | 3,886,718 |  |  |  |  |  |  |  |  |  |  |
Ordinary Shares |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Preferred stock, equivalent ordinary shares upon future consolidation outstanding |  |  |  |  |  |  |  | 0 |  |  |  |  |  |
Convertible senior notes, total ADS into which the debt is exchangeable |  |  |  |  |  |  |  | 28,931,746 |  | 3,886,718 | 6,283,333 |  |  |
American Depositary Shares |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Issuance of stock (shares) | 22,222,223 | 19,178,082 |  |  |  |  |  |  |  |  |  |  |  |
American Depositary Shares | Series A Preferred Stock |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Issuance of stock (shares) |  |  |  |  | 352,150,790 |  |  |  |  |  |  |  |  |
Stock issued under private placement, value | $ |  |  |  |  | $ 52,800,000 |  |  |  |  |  |  |  |  |
American Depositary Shares | Series A Preferred Stock | Second Private Placement |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Issuance of stock (shares) |  |  | 38,867,180 |  |  |  |  |  |  |  |  |  |  |
Stock issued under private placement, value | $ |  |  |  | $ 5,800,000 |  |  |  |  |  |  |  |  |  |
Aggregate gross proceeds | $ |  |  | $ 5,800,000 |  |  |  |  |  |  |  |  |  |  |
Preferred Shares Basis |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share conversion price | $ / shares |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 0.15
Preferred Shares Basis | Series A Preferred Stock | Second Private Placement |  |  |  |  |  |  |  |  |  |  |  |  |  |
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
Ordinary Shares Basis | Series A Preferred Stock | Second Private Placement |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share conversion price | $ / shares |  |  | $ 1.50 |  |  |  |  |  |  |  |  |  |  |

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