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Equity - Preferred Stock - Additional Information (Detail)
 1 Months Ended 3 Months Ended 6 Months Ended
Jun. 30, 2020
£ / shares
shares
Mar. 05, 2015
USD ($)
shares
Jul. 30, 2020
shares
Apr. 30, 2020
shares
Mar. 31, 2020
Mar. 31, 2015
USD ($)
Jun. 30, 2019
Dec. 31, 2019
£ / shares
shares
Mar. 05, 2015
£ / shares
shares
Mar. 05, 2015
$ / shares
shares
Stockholders Equity Note [Line Items]                    
Preferred stock, par value | £ / shares £ 0.05             £ 0.05 £ 0.05  
Private placement, closing date   Mar. 30, 2015                
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                 0.1 0.1
Preferred stock, equivalent ordinary shares upon future consolidation outstanding 5,160,378             28,931,746    
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      
Conversion of stock, shares converted 300,547,010     237,713,680            
Series A Preferred Stock | Subsequent Event                    
Stockholders Equity Note [Line Items]                    
Conversion of stock, shares converted     27,753,000              
Series A Preferred Stock | Minimum                    
Stockholders Equity Note [Line Items]                    
Beneficial ownership limitation   4.99%     9.90%          
Series A Preferred Stock | Maximum                    
Stockholders Equity Note [Line Items]                    
Beneficial ownership limitation   19.90%                
Ordinary Shares                    
Stockholders Equity Note [Line Items]                    
Conversion of Series A Convertible Preferred Stock, net (in shares) 30,054,701     23,771,368            
Preferred stock, equivalent ordinary shares upon future consolidation outstanding 5,160,378                  
Ordinary Shares | Subsequent Event                    
Stockholders Equity Note [Line Items]                    
Conversion of Series A Convertible Preferred Stock, net (in shares)     2,775,300              
Preferred stock, equivalent ordinary shares upon future consolidation outstanding     2,385,078              
American Depositary Shares | Series A Preferred Stock                    
Stockholders Equity Note [Line Items]                    
Issuance of stock (shares)   352,150,790                
Stock issued under private placement, value | $   $ 52,800,000                
Preferred Shares Basis                    
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