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Equity - Additional Information (Detail) $ / shares in Units, $ in Thousands |  |  |  |  |  |  |  | 1 Months Ended | 3 Months Ended | 9 Months Ended |  |  |
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Jul. 11, 2016 shares | Feb. 01, 2016 shares | Jul. 10, 2015 USD ($) $ / shares shares | Jul. 06, 2015 shares | Mar. 30, 2015 USD ($) | Mar. 05, 2015 USD ($) $ / shares shares | Jan. 29, 2015 shares | Sep. 30, 2016 USD ($) $ / shares shares | Aug. 31, 2016 USD ($) $ / shares shares | Nov. 30, 2015 USD ($) | May 31, 2014 USD ($) | Sep. 30, 2016 USD ($) $ / shares shares | Sep. 30, 2015 USD ($) | Jun. 30, 2015 USD ($) shares | Mar. 31, 2015 USD ($) | Sep. 30, 2016 USD ($) $ / shares shares | Sep. 30, 2015 USD ($) shares | Sep. 30, 2016 £ / shares shares | Dec. 31, 2015 £ / shares shares | Mar. 05, 2015 £ / shares
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Convertible senior notes, principal amount | $ |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 128,115 |  |  |  |
Net proceeds from public offering | $ |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 64,614 |  |  |  |
Preferred stock, par value | £ / shares |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | £ 0.05 | £ 0.05 |
Private placement, closing date |  |  |  |  |  | Mar. 30, 2015 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stock issued under private placement, value | $ |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 64,614 |  |  |  |
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 |  |  |  | 32,818,464 |  | 32,818,464 | 32,818,464 |
Market value, per share | $ / shares |  |  |  |  |  | $ 2.39 |  |  |  |  |  |  |  |  |  |  |  |  |  |
BCF resulting from issuance of Series A Preference Shares | $ |  |  |  |  |  |  |  |  |  |  |  |  | $ 1,646 | $ 31,300 |  |  | $ 32,987 |  |  |
Transfer derivative liability to equity | $ |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 1,814 |  |  |
Stock options, outstanding |  |  |  |  |  |  |  | 21,240,894 |  |  |  | 21,240,894 |  |  |  | 21,240,894 |  | 21,240,894 |  |
Restricted stock units, outstanding |  |  |  |  |  |  |  | 10,346,162 |  |  |  | 10,346,162 |  |  |  | 10,346,162 |  | 10,346,162 |  |
Shares issued, exercise of stock options |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 94,272 | 18,020 |  |  |
Proceeds from exercise of stock options, net of transaction costs | $ |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 136 | $ 31 |  |  |
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 |  |  |  | 0 |  |  |  | 0 |  | 0 |  |
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] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Exchange of exchangeable senior notes, net of transaction costs (shares) |  |  |  |  |  |  |  |  |  |  |  |  |  | 6,283,333 |  |  |  |  |  |
Preferred stock, equivalent ordinary shares upon future consolidation outstanding |  |  |  |  |  |  |  | 32,818,464 |  |  |  | 32,818,464 |  |  |  | 32,818,464 |  | 32,818,464 |  |
American Depositary Shares |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Additional shares to be issued |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 24,265,000 |  |  |  |
Net proceeds from public offering | $ |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 64,614 |  |  |  |
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 | $ |  |  |  |  | $ 1,800 | $ 900 |  |  |  |  |  |  |  |  |  |  |  |  |  |
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 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Convertible Debt |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Convertible senior notes, principal amount | $ |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 150,000 |  |  |  |
Convertible Debt | American Depositary Shares |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Exchange of exchangeable senior notes, net of transaction costs (shares) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 60,311,188 |  |  |  |
Convertible Debt | 2014 Notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Convertible senior notes, principal amount | $ |  |  |  |  |  |  |  | $ 118,700 |  |  | $ 118,700 | $ 118,700 |  |  |  |  |  |  |  |
Derivative liability at fair value | $ |  |  |  |  |  |  |  |  |  |  | $ 3,500 |  |  |  |  |  |  |  |  |
Convertible Debt | 2015 Notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Convertible senior notes, principal amount | $ |  |  |  |  |  |  |  |  |  | $ 31,300 |  | $ 31,300 |  |  |  |  |  |  |  |
Derivative liability at fair value | $ |  |  |  |  |  |  |  |  |  | $ 500 |  |  |  |  |  |  |  |  |  |
Final As-Adjusted | Convertible Debt | 2014 Notes | American Depositary Shares |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Exchange of exchangeable senior notes, net of transaction costs (shares) |  |  |  |  |  |  |  |  |  |  |  | 47,739,925 |  |  |  |  |  |  |  |
Final As-Adjusted | Convertible Debt | 2015 Notes | American Depositary Shares |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Exchange of exchangeable senior notes, net of transaction costs (shares) |  |  |  |  |  |  |  |  |  |  |  | 12,571,263 |  |  |  |  |  |  |  |
American Depositary Shares |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Additional shares to be issued |  |  |  |  |  |  |  |  | 21,100,000 |  |  |  |  |  |  |  |  |  |  |
Over allotment option, exercisable period |  |  |  |  |  |  |  |  | 30 days |  |  |  |  |  |  |  |  |  |  |
Sale of stock price per share | $ / shares |  |  |  |  |  |  |  |  | $ 2.679 |  |  |  |  |  |  |  |  |  |  |
Net proceeds from public offering | $ |  |  |  |  |  |  |  |  | $ 64,600 |  |  |  |  |  |  |  |  |  |  |
American Depositary Shares | Series A Preferred Stock |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Additional shares to be issued |  |  |  |  |  | 352,150,790 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stock issued under private placement, value | $ |  |  |  |  |  | $ 52,800 |  |  |  |  |  |  |  |  |  |  |  |  |  |
American Depositary Shares | Second Private Placement | Series A Preferred Stock |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Additional shares to be issued |  |  | 38,867,180 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stock issued under private placement, value | $ |  |  | $ 5,800 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
American Depositary Shares | Over-Allotment Option |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Additional shares to be issued |  |  |  |  |  |  |  |  | 3,165,000 |  |  |  |  |  |  |  |  |  |  |
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 |  |  |  | $ 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 |  |  |  |  |  |  |  | 6.00% |  |  |  | 6.00% |  |  |  | 6.00% |  | 6.00% |  |
Stock Options | Stock Incentive Plan 2011 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stock options, Granted | 208,340 | 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 |  |  |  |  |  |  |  | 3.00% |  |  |  | 3.00% |  |  |  | 3.00% |  | 3.00% |  |
Restricted Stock Units (RSUs) | Stock Incentive Plan 2011 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stock units, Granted | 148,403 | 1,607,500 |  | 1,455,000 |  |  | 2,564,251 |  |  |  |  |  |  |  |  |  |  |  |  |
Stock units, Vesting Period | 3 years | 3 years |  | 4 years |  |  | 3 years |  |  |  |  |  |  |  |  |  |  |  |  |
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 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 272,812 |  |  |  |
Treasury shares for settlement of employee tax obligations |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 125,484 |  |  |  |
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 | 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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