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Equity - Additional Information (Detail)
$ / shares in Units, $ in Thousands
 3 Months Ended 6 Months Ended 9 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
Jun. 30, 2015
USD ($)
shares
Mar. 31, 2015
USD ($)
Jun. 30, 2016
USD ($)
$ / shares
shares
Jun. 30, 2015
USD ($)
shares
Sep. 30, 2015
USD ($)
Jun. 30, 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               20,641,123          
Restricted stock units, outstanding               10,361,147          
Shares issued, exercise of stock options               83,020 18,020        
Proceeds from exercise of stock options, net of transaction costs | $               $ 124 $ 31        
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 | $           $ 31,341     31,341        
Transfer derivative liability to equity | $                 1,814        
Preferred Stock Purchase Option                          
Stockholders Equity Note [Line Items]                          
Derivative liability at fair value | $       $ 900                  
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               181,874          
Treasury shares for settlement of employee tax obligations               77,405          
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