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Equity - Preferred Stock - Additional Information (Detail) |  |  |  | 1 Months Ended | 3 Months Ended | 6 Months Ended |  |
--- | --- | --- | --- | --- | --- | --- | --- | ---
Sep. 30, 2020 £ / shares shares | Jul. 18, 2019 shares | Mar. 05, 2015 USD ($) shares | Jul. 30, 2020 shares | Apr. 30, 2020 shares | Mar. 31, 2020 | Sep. 30, 2019 USD ($) | Mar. 31, 2015 USD ($) | Jun. 30, 2019 | Dec. 31, 2019 £ / shares shares | Mar. 05, 2015 £ / shares shares | Mar. 05, 2015 $ / shares shares
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
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 |  |  |  |  |
Aggregate gross proceeds | $ |  |  |  |  |  |  |  | $ 52,800,000 |  |  |  |
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 | 2,385,078 |  |  |  |  |  |  |  |  | 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 |  |  |
Conversion of stock, shares converted | 328,300,010 |  |  | 27,753,000 | 237,713,680 |  |  |  |  |  |  |
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% |  |  |  |  |  |  |  |  |
Ordinary Shares |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Conversion of Series A Convertible Preferred Stock, net (in shares) | 32,830,001 |  |  | 2,775,300 | 23,771,368 |  |  |  |  |  |  |
Preferred stock, equivalent ordinary shares upon future consolidation outstanding | 2,385,078 |  |  |  |  |  |  |  |  |  |  |
American Depositary Shares |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Issuance of stock (shares) |  | 22,222,223 |  |  |  |  |  |  |  |  |  |
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 |  |  |  |  |  |  |  |  |
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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