Equity - Preferred Stock - Additional Information (Detail)				3 Months Ended				12 Months Ended
	Nov. 29, 2018 shares	Feb. 01, 2018 shares	Mar. 05, 2015 USD ($) shares	Mar. 31, 2019 £ / shares shares	Mar. 31, 2018 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2018 £ / shares shares	Dec. 31, 2015 shares	Mar. 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 | $					$ 70,007,000
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
Market value, per share | $ / shares												$ 2.39
BCF resulting from issuance of Series A Preference Shares | $						$ 31,300,000
Preferred stock, equivalent ordinary shares upon future consolidation outstanding				28,931,746				28,931,746		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%
Series A Preferred Stock | Maximum
Stockholders Equity Note [Line Items]
Beneficial ownership limitation			19.90%
Ordinary Shares
Stockholders Equity Note [Line Items]
Convertible senior notes, total ADS into which the debt is exchangeable								3,886,718	6,283,333
Preferred stock, equivalent ordinary shares upon future consolidation outstanding								28,931,746
American Depositary Shares
Stockholders Equity Note [Line Items]
Issuance of stock (shares)	11,111,112	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
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		$ 1.50