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Equity - Additional Information (Detail)
$ / shares in Units, $ in Thousands
 3 Months Ended
Feb. 01, 2016
shares
Jul. 10, 2015
USD ($)
$ / shares
shares
Jul. 06, 2015
shares
Mar. 05, 2015
USD ($)
$ / shares
shares
Jan. 29, 2015
shares
Mar. 31, 2016
USD ($)
$ / shares
shares
Jun. 30, 2015
shares
Mar. 31, 2015
USD ($)
shares
Mar. 31, 2016
£ / shares
Dec. 31, 2015
£ / shares
shares
Mar. 05, 2015
£ / shares
Stockholders Equity Note [Line Items]                      
Shares issued, exercise of warrants               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      
Warrants outstanding           0          
Stock options, outstanding           20,806,897          
Restricted stock units, outstanding           10,672,834          
Shares issued, exercise of stock options           21,369   2,114      
Proceeds from exercise of stock options, net of transaction costs | $           $ 22   $ 4      
Preferred stock, 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 £ 0.50
Number of shares issued for each preference shares       0.1              
Preferred stock, equivalent ordinary shares upon future consolidation outstanding           32,818,464       32,818,464  
Market value, per share | $ / shares       $ 2.39              
BCF resulting from issuance of Series A Preference Shares | $               31,300      
Transfer derivative liability to equity | $               1,814      
Preferred Stock Purchase Option                      
Stockholders Equity Note [Line Items]                      
Derivative liability at fair value | $       $ 900              
Transfer derivative liability to equity | $               $ 1,814      
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              
Preferred stock, par value | £ / shares                     £ 0.05
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%              
Ordinary Shares                      
Stockholders Equity Note [Line Items]                      
Shares issued             6,283,333        
Preferred stock, equivalent ordinary shares upon future consolidation outstanding           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          
Derivative liability at fair value | $       $ 900   1,800          
Gain (loss) on change in fair value of derivatives | $       $ (900)   (1,600)          
Transfer derivative liability to equity | $           $ 1,800          
Second Private Placement | Series A Preferred Stock                      
Stockholders Equity Note [Line Items]                      
Preferred stock, equivalent ordinary shares upon future consolidation outstanding   3,886,718                  
American Depository Shares | Series A Preferred Stock                      
Stockholders Equity Note [Line Items]                      
Stock issued under private placement, shares       352,150,790              
Stock issued under private placement, value | $       $ 52,800              
American Depository Shares | Second Private Placement | Series A Preferred Stock                      
Stockholders Equity Note [Line Items]                      
Stock issued under private placement, shares   38,867,180                  
Stock issued under private placement, value | $   $ 5,800                  
Preferred Shares Basis                      
Stockholders Equity Note [Line Items]                      
Share conversion price | $ / shares       $ 0.15              
Preferred Shares Basis | Second Private Placement | Series A Preferred Stock                      
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 | Second Private Placement | Series A Preferred Stock                      
Stockholders Equity Note [Line Items]                      
Share conversion price | $ / shares   $ 1.50                  
Stock Options                      
Stockholders Equity Note [Line Items]                      
Percentage of outstanding shares on a fully diluted basis           8.00%          
Stock Options | Stock Incentive Plan 2011                      
Stockholders Equity Note [Line Items]                      
Stock options, Granted 2,442,000   5,470,000   1,622,500            
Stock units, Vesting Period 4 years       4 years            
Stock Options | Stock Incentive Plan 2011 | Vest over a four year period                      
Stockholders Equity Note [Line Items]                      
Stock options, Granted     3,670,000                
Stock units, Vesting Period     4 years                
Stock Options | Stock Incentive Plan 2011 | Director                      
Stockholders Equity Note [Line Items]                      
Stock options, Granted     288,657                
Stock Options | Stock Incentive Plan 2011 | Director | Vest in equal installments over a three year period                      
Stockholders Equity Note [Line Items]                      
Stock options, Granted     121,506                
Stock units, Vesting Period     3 years                
Stock Options | Stock Incentive Plan 2011 | Director | Vest in full                      
Stockholders Equity Note [Line Items]                      
Stock options, Granted     167,151                
Restricted Stock Units (RSUs)                      
Stockholders Equity Note [Line Items]                      
Percentage of outstanding shares on a fully diluted basis           4.00%          
Restricted Stock Units (RSUs) | Stock Incentive Plan 2011                      
Stockholders Equity Note [Line Items]                      
Stock units, Granted 1,607,500   1,455,000   2,564,251            
Stock units, Vesting Period 3 years   4 years   3 years            
Restricted Stock Units (RSUs) | Stock Incentive Plan 2011 | Vest in equal installments over a three year period                      
Stockholders Equity Note [Line Items]                      
Shares issued, related to the vesting of RSUs           818,352          
Treasury shares for settlement of employee tax obligations           270,329          
Restricted Stock Units (RSUs) | Stock Incentive Plan 2011 | Vest over a four year period                      
Stockholders Equity Note [Line Items]                      
Shares issued, related to the vesting of RSUs           90,937          
Treasury shares for settlement of employee tax obligations           34,801          
Restricted Stock Units (RSUs) | Stock Incentive Plan 2011 | Director                      
Stockholders Equity Note [Line Items]                      
Stock units, Granted     413,500                
Restricted Stock Units (RSUs) | Stock Incentive Plan 2011 | Director | Vest in equal installments over a three year period                      
Stockholders Equity Note [Line Items]                      
Stock units, Granted     283,500                
Stock units, Vesting Period     3 years                
Restricted Stock Units (RSUs) | Stock Incentive Plan 2011 | Director | Vest in full                      
Stockholders Equity Note [Line Items]                      
Stock units, Granted     130,000                
Performance Based Options | Stock Incentive Plan 2011 | Vest upon the achievement of certain performance conditions                      
Stockholders Equity Note [Line Items]                      
Stock options, Granted     1,800,000                
Performance Based Restricted Stock Unit | Stock Incentive Plan 2011                      
Stockholders Equity Note [Line Items]                      
Stock units, Granted         5,455,500