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Equity - Additional Information (Detail) $ / shares in Units, $ in Thousands |  |  |  |  |  |  | 3 Months Ended | 9 Months Ended |  |  |
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Jul. 10, 2015 USD ($) $ / shares shares | Jul. 06, 2015 shares | Mar. 05, 2015 USD ($) $ / shares shares | Jan. 29, 2015 shares | Mar. 11, 2014 shares | Jan. 08, 2014 shares | Sep. 30, 2015 USD ($) $ / shares shares | Sep. 30, 2014 USD ($) | Sep. 30, 2015 USD ($) $ / shares shares | Sep. 30, 2014 USD ($) shares | Sep. 30, 2015 £ / shares | Mar. 05, 2015 £ / shares | Dec. 31, 2014 £ / shares shares
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Shares issued, exercise of warrants |  |  |  |  |  |  |  |  | 1,844,585 | 1,684,888 |  |  |
Proceeds from exercise of warrants, gross of transaction costs | $ |  |  |  |  |  |  |  |  | $ 2,800 | $ 1,700 |  |  |
Proceeds from exercise of warrants, net of transaction costs | $ |  |  |  |  |  |  |  |  | $ 2,713 | $ 1,651 |  |  |
Stock options, outstanding |  |  |  |  |  |  | 17,626,803 |  | 17,626,803 |  |  |  |
Restricted stock units, outstanding |  |  |  |  |  |  | 11,001,361 |  | 11,001,361 |  |  |  |
Shares issued, exercise of stock options |  |  |  |  |  |  |  |  | 18,020 | 234,500 |  |  |
Proceeds from exercise of stock options, net of transaction costs | $ |  |  |  |  |  |  |  |  | $ 31 | $ 307 |  |  |
Treasury shares |  |  |  |  |  |  | 152,071 |  | 152,071 |  |  |  | 20,079
Tax refund received | $ |  |  |  |  |  |  |  | $ 3,191 |  | $ 3,191 |  |  |
Preference Shares, par value | £ / shares |  |  |  |  |  |  |  |  |  |  | £ 0.05 |  |
Private placement, closing date |  |  | Mar. 30, 2015 |  |  |  |  |  |  |  |  |  |
Stock issued under private placement, value | $ |  |  |  |  |  |  |  |  | $ 57,853 |  |  |  |
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 |  |  |  |  |  |  | 0.1 |  | 0.1 |  |  |  |
Preferred stock, equivalent ordinary shares upon future consolidation |  |  |  |  |  |  | 32,818,464 |  | 32,818,464 |  |  |  | 0
Market value, per share | $ / shares |  |  | $ 2.39 |  |  |  |  |  |  |  |  |  |
BCF resulting from issuance of Series A Preference Shares | $ |  |  |  |  |  |  | $ 1,646 |  | $ 32,987 |  |  |  |
Transfer derivative liability to equity | $ |  |  |  |  |  |  |  |  | $ 1,814 |  |  |  |
Minimum |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Beneficial ownership limitation |  |  |  |  |  |  |  |  | 4.99% |  |  |  |
Maximum |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Beneficial ownership limitation |  |  |  |  |  |  |  |  | 19.90% |  |  |  |
American Depository Shares |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Stock issued under private placement, shares |  |  | 352,150,790 |  |  |  |  |  |  |  |  |  |
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 |
Shares issued | 38,867,180 |  |  |  |  |  |  |  |  |  |  |  |
Preferred stock, equivalent ordinary shares upon future consolidation | 3,886,718 |  |  |  |  |  |  |  |  |  |  |  |
Subscription agreement to purchase Additional shares | $ | $ 5,800 |  |  |  |  |  |  |  |  |  |  |  |
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 |  |  |  |  |  |  |  |  | 6,283,333 |  |  |  |
Preferred stock, equivalent ordinary shares upon future consolidation |  |  |  |  |  |  | 32,818,464 |  | 32,818,464 |  |  |  |
Preferred Stock Purchase Option |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative liability at fair value | $ |  |  | $ 900 |  |  |  |  |  |  |  |  |  |
Transfer derivative liability to equity | $ |  |  |  |  |  |  |  |  | $ 1,814 |  |  |  |
Preferred Stock Purchase Option | First Private Placement |  |  |  |  |  |  |  |  |  |  |  |  |
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 |  |  |  |
Preferred Stock Purchase Option | Second Private Placement |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
BCF resulting from issuance of Series A Preference Shares | $ |  |  |  |  |  |  |  |  | 3,400 |  |  |  |
Derivative liability at fair value | $ |  |  |  |  |  |  | $ 1,800 |  | 1,800 |  |  |  |
Gain (loss) on change in fair value of derivatives | $ |  |  |  |  |  |  |  |  | $ (1,600) |  |  |  |
Stock Incentive Plan 2011 |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Shares issued, related to the vesting of RSUs |  |  |  |  |  |  |  |  | 639,500 |  |  |  |
Stock Incentive Plan 2011 | Employee tax obligations |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Treasury shares |  |  |  |  |  |  | 114,258 |  | 114,258 |  |  |  |
Preferred Shares Basis |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Share conversion price | $ / shares |  |  |  |  |  |  | $ 0.15 |  | $ 0.15 |  |  |  |
Preferred Shares Basis | 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 | Series A Preferred Stock |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Share conversion price | $ / shares | $ 1.50 |  |  |  |  |  |  |  |  |  |  |  |
Restricted Stock Units (RSUs) |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of outstanding shares on a fully diluted basis |  |  |  |  |  |  |  |  | 7.00% |  |  |  |
Restricted Stock Units (RSUs) | Stock Incentive Plan 2011 |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Stock units, Granted |  | 1,455,000 |  | 2,564,251 | 173,348 | 2,082,000 |  |  |  |  |  |  |
Stock options, Vesting Period |  | 4 years |  | 3 years | 3 years | 3 years |  |  |  |  |  |  |
Restricted Stock Units (RSUs) | Stock Incentive Plan 2011 | Director [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Stock units, Granted |  | 413,500 |  |  |  |  |  |  |  |  |  |  |
Restricted Stock Units (RSUs) | Stock Incentive Plan 2011 | Director [Member] | Vest in equal installments over a three year period upon the earlier of the one-year anniversary of the grant date or the Company's annual general meeting of shareholders in such anniversary year |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Stock options, Vesting Period |  | 3 years |  |  |  |  |  |  |  |  |  |  |
Stock units, expected to vest |  | 283,500 |  |  |  |  |  |  |  |  |  |  |
Restricted Stock Units (RSUs) | Stock Incentive Plan 2011 | Director [Member] | Vest in full upon the earlier of the one-year anniversary of the grant date or the Company's annual general meeting of shareholders in such anniversary year |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Stock units, expected to vest |  | 130,000 |  |  |  |  |  |  |  |  |  |  |
Stock Options |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of outstanding shares on a fully diluted basis |  |  |  |  |  |  |  |  | 4.00% |  |  |  |
Stock Options | Stock Incentive Plan 2011 |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Granted options |  | 5,470,000 |  | 1,622,500 | 205,890 | 2,605,500 |  |  |  |  |  |  |
Stock options, Vesting Period |  | 4 years |  | 4 years |  | 4 years |  |  |  |  |  |  |
Stock options, expected to vest |  | 3,670,000 |  |  |  |  |  |  |  |  |  |  |
Stock Options | Stock Incentive Plan 2011 | Director [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Granted options |  | 288,657 |  |  |  |  |  |  |  |  |  |  |
Stock Options | Stock Incentive Plan 2011 | Director [Member] | Vest in equal installments over a three year period upon the earlier of the one-year anniversary of the grant date or the Company's annual general meeting of shareholders in such anniversary year |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Stock options, Vesting Period |  | 3 years |  |  |  |  |  |  |  |  |  |  |
Stock options, expected to vest |  | 121,506 |  |  |  |  |  |  |  |  |  |  |
Stock Options | Stock Incentive Plan 2011 | Director [Member] | Vest in full upon the earlier of the one-year anniversary of the grant date or the Company's annual general meeting of shareholders in such anniversary year |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Stock options, expected to vest |  | 167,151 |  |  |  |  |  |  |  |  |  |  |
Performance Based Options | Stock Incentive Plan 2011 |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Stock options, expected to vest |  | 1,800,000 |  |  |  |  |  |  |  |  |  |  |
Shares issued, related to the vesting of RSUs |  |  |  |  |  |  |  |  | 84,688 |  |  |  |
Performance Based Options | Stock Incentive Plan 2011 | Employee tax obligations |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Treasury shares |  |  |  |  |  |  | 17,734 |  | 17,734 |  |  |  |
Performance Based Restricted Stock Unit | Stock Incentive Plan 2011 |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Stock units, Granted |  |  |  | 5,455,500 |  |  |  |  |  |  |  |  |

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