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Equity - Preferred Stock - Additional Information (Detail)
 3 Months Ended 9 Months Ended 12 Months Ended
Feb. 01, 2018
shares
Jul. 10, 2015
$ / shares
shares
Mar. 30, 2015
USD ($)
Mar. 05, 2015
USD ($)
$ / shares
shares
Sep. 30, 2018
£ / shares
shares
Jun. 30, 2015
USD ($)
Mar. 31, 2015
USD ($)
Sep. 30, 2018
USD ($)
Dec. 31, 2015
USD ($)
shares
Sep. 30, 2018
$ / shares
shares
Dec. 31, 2017
£ / shares
shares
Mar. 05, 2015
£ / shares
Stockholders Equity Note [Line Items]                        
Preferred stock, par value | £ / shares         £ 0.05           £ 0.05 £ 0.05
Private placement, closing date       Mar. 30, 2015                
Stock issued under private placement, value               $ 70,007,000        
Aggregate gross proceeds             $ 52,800,000          
Estimated offering expense             $ 700,000          
Common stock, par value | £ / shares         £ 0.50           £ 0.50 £ 0.50
Number of shares issued for each preference shares | shares       0.1                
Market value, per share | $ / shares       $ 2.39                
BCF resulting from issuance of Series A Preference Shares           $ 31,300,000            
Preferred stock, equivalent ordinary shares upon future consolidation outstanding | shares         28,931,746         28,931,746 32,818,464  
Second Private Placement | Preferred Stock Purchase Option                        
Stockholders Equity Note [Line Items]                        
BCF resulting from issuance of Series A Preference Shares                 $ 3,400,000      
Derivative liability at fair value     $ 1,800,000 $ 900,000                
Gain (loss) on change in fair value of derivatives       $ (900,000)         $ (1,600,000)      
Transfer derivative liability to equity     1,800,000                  
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        
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%                
Series A Preferred Stock | Maximum | Second Private Placement                        
Stockholders Equity Note [Line Items]                        
Preferred stock, equivalent ordinary shares upon future consolidation outstanding | shares   3,886,718                    
Ordinary Shares                        
Stockholders Equity Note [Line Items]                        
Convertible senior notes, total ADS into which the debt is exchangeable | shares         3,886,718       6,283,333      
Preferred stock, equivalent ordinary shares upon future consolidation outstanding | shares         28,931,746         28,931,746    
American Depositary Shares                        
Stockholders Equity Note [Line Items]                        
Issuance of stock (shares) | shares 19,178,082                      
American Depositary Shares | Series A Preferred Stock                        
Stockholders Equity Note [Line Items]                        
Issuance of stock (shares) | shares       352,150,790                
Stock issued under private placement, value       $ 52,800,000                
American Depositary Shares | Series A Preferred Stock | Second Private Placement                        
Stockholders Equity Note [Line Items]                        
Issuance of stock (shares) | shares   38,867,180                    
Stock issued under private placement, value     $ 5,800,000                  
Preferred Shares Basis                        
Stockholders Equity Note [Line Items]                        
Share conversion price | $ / shares       $ 0.15                
Preferred Shares Basis | Series A Preferred Stock | Second Private Placement                        
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 | Second Private Placement                        
Stockholders Equity Note [Line Items]                        
Share conversion price | $ / shares   $ 1.50