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Equity - Additional Information (Detail)
$ / shares in Units, $ in Thousands
 3 Months Ended 9 Months Ended
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