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Equity - Common Stock - Additional Information (Detail)
$ / shares in Units, $ in Millions
 9 Months Ended
Mar. 16, 2020
£ / shares
shares
Jul. 29, 2019
USD ($)
Jul. 18, 2019
$ / shares
shares
Sep. 30, 2020
£ / shares
shares
Sep. 30, 2019
shares
Dec. 31, 2019
£ / shares
Mar. 05, 2015
£ / shares
Stockholders Equity Note [Line Items]              
Common stock, par value | £ / shares       £ 0.50   £ 0.50 £ 0.50
2011 Plan              
Stockholders Equity Note [Line Items]              
Number of shares remaining under the plan       2,634,440      
Amarin Corporation Plc 2017 Employee Stock Purchase Plan | Ordinary Shares              
Stockholders Equity Note [Line Items]              
Issuance of common stock, net of transaction costs (shares)       123,608 47,358    
American Depositary Shares              
Stockholders Equity Note [Line Items]              
Issuance of common stock, net of transaction costs (shares)     22,222,223        
American Depository Shares Ratio     1        
Sale of stock price per share | $ / shares     $ 18.00        
American Depositary Shares | Over-Allotment Option              
Stockholders Equity Note [Line Items]              
Sale of stock price per share | $ / shares     $ 18.00        
Issuance of stock (shares), exercised     3,333,333        
Gross proceeds from public offering including exercised option | $   $ 460.0          
Net proceeds from public offering including exercised option | $   $ 440.1          
American Depositary Shares | Stock Incentive Plan 2020              
Stockholders Equity Note [Line Items]              
Expiration date of stock incentive plan 2030-07            
American Depositary Shares | Stock Incentive Plan 2020 | Ordinary Shares              
Stockholders Equity Note [Line Items]              
Common stock, par value | £ / shares £ 0.50            
Issuance of common stock, net of transaction costs (shares) 20,000,000