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Equity - Additional Information (Detail) $ / shares in Units, $ in Thousands |  |  |  |  | 3 Months Ended | 6 Months Ended |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Mar. 05, 2015 USD ($) $ / shares | Jan. 29, 2015 shares | Mar. 11, 2014 shares | Jan. 08, 2014 shares | Jun. 30, 2015 USD ($) $ / shares shares | Jun. 30, 2015 USD ($) $ / shares shares | Jun. 30, 2014 USD ($) shares | Jun. 30, 2015 £ / shares shares | Mar. 30, 2015 USD ($) | Mar. 05, 2015 £ / shares | Dec. 31, 2014 £ / shares shares
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |
Shares issued, exercise of warrants | shares |  |  |  |  |  | 1,844,585 |  |  |  |  |
Proceeds from exercise of warrants, gross of transaction costs |  |  |  |  |  | $ 2,800 |  |  |  |  |
Proceeds from exercise of warrants, net of transaction costs |  |  |  |  |  | $ 2,713 |  |  |  |  |
Shares issued, exercise of stock options | shares |  |  |  |  |  | 18,020 | 215,000 |  |  |  |
Proceeds from exercise of stock options, net of transaction costs |  |  |  |  |  | $ 31 | $ 284 |  |  |  |
Shares issued, related to the vesting of RSUs | shares |  |  |  |  |  | 639,500 |  |  |  |  |
Treasury shares | shares |  |  |  |  | 134,337 | 134,337 |  | 134,337 |  |  | 20,079
Stock issued under private placement, value |  |  |  |  |  | $ 52,149 |  |  |  |  |
Preference Shares, par value | £ / shares |  |  |  |  |  |  |  | £ 0.05 |  |  | £ 0.05
Private placement, closing date | Mar. 30, 2015 |  |  |  |  |  |  |  |  |  |
Aggregate gross proceeds |  |  |  |  |  | 52,800 |  |  |  |  |
Estimated offering expense |  |  |  |  |  | $ 700 |  |  |  |  |
Common stock, par value | £ / shares |  |  |  |  |  |  |  | £ 0.50 |  |  | £ 0.50
Number of shares issued for each preference shares | shares |  |  |  |  | 0.1 | 0.1 |  | 0.1 |  |  |
Preferred stock, equivalent ordinary shares upon future consolidation | shares |  |  |  |  | 28,931,746 | 28,931,746 |  | 28,931,746 |  |  | 0
Market value, per share | $ / shares | $ 2.39 |  |  |  |  |  |  |  |  |  |
BCF resulting from issuance of Series A Preference Shares |  |  |  |  | $ 31,341 | $ 31,341 |  |  |  |  |
Transfer derivative liability to equity |  |  |  |  |  | 1,814 |  |  |  |  |
Preferred Stock Purchase Option |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |
Derivative liability at fair value | $ 900 |  |  |  |  |  |  |  |  |  |
Gain (loss) on change in fair value of derivatives | (900) |  |  |  |  |  |  |  |  |  |
Transfer derivative liability to equity |  |  |  |  |  | $ 1,814 |  |  |  |  |
American Depository Shares |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |
Stock issued under private placement, value | $ 52,800 |  |  |  |  |  |  |  |  |  |
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 |  |  |  |  |  |  |  |  |  |
Preference Shares, par value | £ / shares |  |  |  |  |  |  |  |  |  | £ 0.05 |
Subscription agreement to purchase Additional shares |  |  |  |  |  |  |  |  | $ 5,800 |  |
Preferred Shares Basis |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |
Share conversion price | $ / shares |  |  |  |  | $ 0.15 | $ 0.15 |  |  |  |  |
Ordinary Shares Basis |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |
Share conversion price | $ / shares |  |  |  |  | $ 1.50 | $ 1.50 |  |  |  |  |
Ordinary Shares |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |
Shares issued | shares |  |  |  |  |  | 6,283,333 |  |  |  |  |
Preferred stock, equivalent ordinary shares upon future consolidation | shares |  |  |  |  | 28,931,746 | 28,931,746 |  | 28,931,746 |  |  |
Employee tax obligations |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |
Treasury shares | shares |  |  |  |  | 114,258 | 114,258 |  | 114,258 |  |  |
Minimum |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |
Beneficial ownership limitation |  |  |  |  |  | 4.99% |  |  |  |  |
Maximum |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |
Beneficial ownership limitation |  |  |  |  |  | 19.90% |  |  |  |  |
Stock Incentive Plan 2011 | Restricted Stock Units (RSUs) |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |
Stock units, Granted | shares |  | 2,564,251 | 173,348 | 2,082,000 |  |  |  |  |  |  |
Stock options, Vesting Period |  | 3 years | 3 years | 3 years |  |  |  |  |  |  |
Stock Incentive Plan 2011 | Stock Options |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |
Granted options | shares |  | 1,622,500 | 205,890 | 2,605,500 |  |  |  |  |  |  |
Stock options, Vesting Period |  | 4 years | 1 year | 4 years |  |  |  |  |  |  |
Stock Incentive Plan 2011 | Performance Based Restricted Stock Unit |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |
Stock units, Granted | shares |  | 5,455,500 |  |  |  |  |  |  |  |  |

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