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Equity - Additional Information (Detail) $ / shares in Units, $ in Thousands |  |  |  |  |  | 3 Months Ended |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Feb. 01, 2016 shares | Jul. 10, 2015 USD ($) $ / shares shares | Jul. 06, 2015 shares | Mar. 05, 2015 USD ($) $ / shares shares | Jan. 29, 2015 shares | Mar. 31, 2016 USD ($) $ / shares shares | Jun. 30, 2015 shares | Mar. 31, 2015 USD ($) shares | Mar. 31, 2016 £ / shares | Dec. 31, 2015 £ / shares shares | Mar. 05, 2015 £ / shares
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |
Shares issued, exercise of warrants |  |  |  |  |  |  |  | 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 |  |  |
Warrants outstanding |  |  |  |  |  | 0 |  |  |  |  |
Stock options, outstanding |  |  |  |  |  | 20,806,897 |  |  |  |  |
Restricted stock units, outstanding |  |  |  |  |  | 10,672,834 |  |  |  |  |
Shares issued, exercise of stock options |  |  |  |  |  | 21,369 |  | 2,114 |  |  |
Proceeds from exercise of stock options, net of transaction costs | $ |  |  |  |  |  | $ 22 |  | $ 4 |  |  |
Preferred stock, 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 | £ 0.50
Number of shares issued for each preference shares |  |  |  | 0.1 |  |  |  |  |  |  |
Preferred stock, equivalent ordinary shares upon future consolidation outstanding |  |  |  |  |  | 32,818,464 |  |  |  | 32,818,464 |
Market value, per share | $ / shares |  |  |  | $ 2.39 |  |  |  |  |  |  |
BCF resulting from issuance of Series A Preference Shares | $ |  |  |  |  |  |  |  | 31,300 |  |  |
Transfer derivative liability to equity | $ |  |  |  |  |  |  |  | 1,814 |  |  |
Preferred Stock Purchase Option |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |
Derivative liability at fair value | $ |  |  |  | $ 900 |  |  |  |  |  |  |
Transfer derivative liability to equity | $ |  |  |  |  |  |  |  | $ 1,814 |  |  |
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 |  |  |  |  |  |  |
Preferred stock, par value | £ / shares |  |  |  |  |  |  |  |  |  |  | £ 0.05
Series A Preferred Stock | Minimum |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |
Beneficial ownership limitation |  |  |  | 4.99% |  |  |  |  |  |  |
Series A Preferred Stock | Maximum |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |
Beneficial ownership limitation |  |  |  | 19.90% |  |  |  |  |  |  |
Ordinary Shares |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |
Shares issued |  |  |  |  |  |  | 6,283,333 |  |  |  |
Preferred stock, equivalent ordinary shares upon future consolidation outstanding |  |  |  |  |  | 32,818,464 |  |  |  |  |
Second Private Placement | Preferred Stock Purchase Option |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |
BCF resulting from issuance of Series A Preference Shares | $ |  |  |  |  |  | $ 3,400 |  |  |  |  |
Derivative liability at fair value | $ |  |  |  | $ 900 |  | 1,800 |  |  |  |  |
Gain (loss) on change in fair value of derivatives | $ |  |  |  | $ (900) |  | (1,600) |  |  |  |  |
Transfer derivative liability to equity | $ |  |  |  |  |  | $ 1,800 |  |  |  |  |
Second Private Placement | Series A Preferred Stock |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |
Preferred stock, equivalent ordinary shares upon future consolidation outstanding |  | 3,886,718 |  |  |  |  |  |  |  |  |
American Depository Shares | Series A Preferred Stock |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |
Stock issued under private placement, shares |  |  |  | 352,150,790 |  |  |  |  |  |  |
Stock issued under private placement, value | $ |  |  |  | $ 52,800 |  |  |  |  |  |  |
American Depository Shares | Second Private Placement | Series A Preferred Stock |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |
Stock issued under private placement, shares |  | 38,867,180 |  |  |  |  |  |  |  |  |
Stock issued under private placement, value | $ |  | $ 5,800 |  |  |  |  |  |  |  |  |
Preferred Shares Basis |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |
Share conversion price | $ / shares |  |  |  | $ 0.15 |  |  |  |  |  |  |
Preferred Shares Basis | Second Private Placement | Series A Preferred Stock |  |  |  |  |  |  |  |  |  |  |
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 |  | $ 1.50 |  |  |  |  |
Ordinary Shares Basis | Second Private Placement | Series A Preferred Stock |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |
Share conversion price | $ / shares |  | $ 1.50 |  |  |  |  |  |  |  |  |
Stock Options |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |
Percentage of outstanding shares on a fully diluted basis |  |  |  |  |  | 8.00% |  |  |  |  |
Stock Options | Stock Incentive Plan 2011 |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |
Stock options, Granted | 2,442,000 |  | 5,470,000 |  | 1,622,500 |  |  |  |  |  |
Stock units, Vesting Period | 4 years |  |  |  | 4 years |  |  |  |  |  |
Stock Options | Stock Incentive Plan 2011 | Vest over a four year period |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |
Stock options, Granted |  |  | 3,670,000 |  |  |  |  |  |  |  |
Stock units, Vesting Period |  |  | 4 years |  |  |  |  |  |  |  |
Stock Options | Stock Incentive Plan 2011 | Director |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |
Stock options, Granted |  |  | 288,657 |  |  |  |  |  |  |  |
Stock Options | Stock Incentive Plan 2011 | Director | Vest in equal installments over a three year period |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |
Stock options, Granted |  |  | 121,506 |  |  |  |  |  |  |  |
Stock units, Vesting Period |  |  | 3 years |  |  |  |  |  |  |  |
Stock Options | Stock Incentive Plan 2011 | Director | Vest in full |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |
Stock options, Granted |  |  | 167,151 |  |  |  |  |  |  |  |
Restricted Stock Units (RSUs) |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |
Percentage of outstanding shares on a fully diluted basis |  |  |  |  |  | 4.00% |  |  |  |  |
Restricted Stock Units (RSUs) | Stock Incentive Plan 2011 |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |
Stock units, Granted | 1,607,500 |  | 1,455,000 |  | 2,564,251 |  |  |  |  |  |
Stock units, Vesting Period | 3 years |  | 4 years |  | 3 years |  |  |  |  |  |
Restricted Stock Units (RSUs) | Stock Incentive Plan 2011 | Vest in equal installments over a three year period |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |
Shares issued, related to the vesting of RSUs |  |  |  |  |  | 818,352 |  |  |  |  |
Treasury shares for settlement of employee tax obligations |  |  |  |  |  | 270,329 |  |  |  |  |
Restricted Stock Units (RSUs) | Stock Incentive Plan 2011 | Vest over a four year period |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |
Shares issued, related to the vesting of RSUs |  |  |  |  |  | 90,937 |  |  |  |  |
Treasury shares for settlement of employee tax obligations |  |  |  |  |  | 34,801 |  |  |  |  |
Restricted Stock Units (RSUs) | Stock Incentive Plan 2011 | Director |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |
Stock units, Granted |  |  | 413,500 |  |  |  |  |  |  |  |
Restricted Stock Units (RSUs) | Stock Incentive Plan 2011 | Director | Vest in equal installments over a three year period |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |
Stock units, Granted |  |  | 283,500 |  |  |  |  |  |  |  |
Stock units, Vesting Period |  |  | 3 years |  |  |  |  |  |  |  |
Restricted Stock Units (RSUs) | Stock Incentive Plan 2011 | Director | Vest in full |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |
Stock units, Granted |  |  | 130,000 |  |  |  |  |  |  |  |
Performance Based Options | Stock Incentive Plan 2011 | Vest upon the achievement of certain performance conditions |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |
Stock options, Granted |  |  | 1,800,000 |  |  |  |  |  |  |  |
Performance Based Restricted Stock Unit | Stock Incentive Plan 2011 |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |
Stock units, Granted |  |  |  |  | 5,455,500 |  |  |  |  |  |

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