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Equity - Preferred Stock - Additional Information (Detail)
 1 Months Ended 3 Months Ended 6 Months Ended
Sep. 30, 2020
£ / shares
shares
Jul. 18, 2019
shares
Mar. 05, 2015
USD ($)
shares
Jul. 30, 2020
shares
Apr. 30, 2020
shares
Mar. 31, 2020
Sep. 30, 2019
USD ($)
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                  
Stock issued under private placement, value | $             $ 440,108,000          
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                     0.1 0.1
Preferred stock, equivalent ordinary shares upon future consolidation outstanding 2,385,078                 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 328,300,010     27,753,000 237,713,680              
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) 32,830,001     2,775,300 23,771,368              
Preferred stock, equivalent ordinary shares upon future consolidation outstanding 2,385,078                      
American Depositary Shares                        
Stockholders Equity Note [Line Items]                        
Issuance of stock (shares)   22,222,223                    
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