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Equity - Additional Information (Detail)
$ / shares in Units, $ in Thousands
 3 Months Ended 6 Months Ended
Mar. 05, 2015
USD ($)
$ / shares
Jan. 29, 2015
shares
Mar. 11, 2014
shares
Jan. 08, 2014
shares
Jun. 30, 2015
USD ($)
$ / shares
shares
Jun. 30, 2015
USD ($)
$ / shares
shares
Jun. 30, 2014
USD ($)
shares
Jun. 30, 2015
£ / shares
shares
Mar. 30, 2015
USD ($)
Mar. 05, 2015
£ / shares
Dec. 31, 2014
£ / shares
shares
Stockholders Equity Note [Line Items]                      
Shares issued, exercise of warrants | shares           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          
Shares issued, exercise of stock options | shares           18,020 215,000        
Proceeds from exercise of stock options, net of transaction costs           $ 31 $ 284        
Shares issued, related to the vesting of RSUs | shares           639,500          
Treasury shares | shares         134,337 134,337   134,337     20,079
Stock issued under private placement, value           $ 52,149          
Preference Shares, 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
Number of shares issued for each preference shares | shares         0.1 0.1   0.1      
Preferred stock, equivalent ordinary shares upon future consolidation | shares         28,931,746 28,931,746   28,931,746     0
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                    
Gain (loss) on change in fair value of derivatives (900)                    
Transfer derivative liability to equity           $ 1,814          
American Depository Shares                      
Stockholders Equity Note [Line Items]                      
Stock issued under private placement, value $ 52,800                    
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                    
Preference Shares, par value | £ / shares                   £ 0.05  
Subscription agreement to purchase Additional shares                 $ 5,800    
Preferred Shares Basis                      
Stockholders Equity Note [Line Items]                      
Share conversion price | $ / shares         $ 0.15 $ 0.15          
Ordinary Shares Basis                      
Stockholders Equity Note [Line Items]                      
Share conversion price | $ / shares         $ 1.50 $ 1.50          
Ordinary Shares                      
Stockholders Equity Note [Line Items]                      
Shares issued | shares           6,283,333          
Preferred stock, equivalent ordinary shares upon future consolidation | shares         28,931,746 28,931,746   28,931,746      
Employee tax obligations                      
Stockholders Equity Note [Line Items]                      
Treasury shares | shares         114,258 114,258   114,258      
Minimum                      
Stockholders Equity Note [Line Items]                      
Beneficial ownership limitation           4.99%          
Maximum                      
Stockholders Equity Note [Line Items]                      
Beneficial ownership limitation           19.90%          
Stock Incentive Plan 2011 | Restricted Stock Units (RSUs)                      
Stockholders Equity Note [Line Items]                      
Stock units, Granted | shares   2,564,251 173,348 2,082,000              
Stock options, Vesting Period   3 years 3 years 3 years              
Stock Incentive Plan 2011 | Stock Options                      
Stockholders Equity Note [Line Items]                      
Granted options | shares   1,622,500 205,890 2,605,500              
Stock options, Vesting Period   4 years 1 year 4 years              
Stock Incentive Plan 2011 | Performance Based Restricted Stock Unit                      
Stockholders Equity Note [Line Items]                      
Stock units, Granted | shares   5,455,500