Equity - Preferred Stock - Additional Information (Detail)						1 Months Ended	3 Months Ended	12 Months Ended
	Jul. 18, 2019 shares	Feb. 01, 2018 shares	Jul. 10, 2015 USD ($) $ / shares shares	Mar. 30, 2015 USD ($)	Mar. 05, 2015 USD ($) shares	Mar. 31, 2020	Mar. 31, 2015 USD ($)	Dec. 31, 2020 £ / shares shares	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($) shares	Dec. 31, 2015 shares	Dec. 31, 2019 £ / shares shares	Mar. 05, 2015 £ / shares shares	Mar. 05, 2015 $ / shares shares
Stockholders Equity Note [Line Items]
Preferred stock, shares outstanding								0				289,317,460
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	$ 264,840,000
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												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
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%
Series A Preferred Stock | Maximum | Second Private Placement
Stockholders Equity Note [Line Items]
Preferred stock, equivalent ordinary shares upon future consolidation outstanding			3,886,718
Ordinary Shares
Stockholders Equity Note [Line Items]
Preferred stock, equivalent ordinary shares upon future consolidation outstanding								0
Convertible senior notes, total ADS into which the debt is exchangeable								28,931,746		3,886,718	6,283,333
American Depositary Shares
Stockholders Equity Note [Line Items]
Issuance of stock (shares)	22,222,223	19,178,082
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
American Depositary Shares | Series A Preferred Stock | Second Private Placement
Stockholders Equity Note [Line Items]
Issuance of stock (shares)			38,867,180
Stock issued under private placement, value | $				$ 5,800,000
Aggregate gross proceeds | $			$ 5,800,000
Preferred Shares Basis
Stockholders Equity Note [Line Items]
Share conversion price | $ / shares														$ 0.15
Preferred Shares Basis | Series A Preferred Stock | Second Private Placement
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
Ordinary Shares Basis | Series A Preferred Stock | Second Private Placement
Stockholders Equity Note [Line Items]
Share conversion price | $ / shares			$ 1.50