Equity - Preferred Stock - Additional Information (Detail)					3 Months Ended				9 Months Ended	12 Months Ended
	Jul. 18, 2019 shares	Nov. 29, 2018 shares	Feb. 01, 2018 shares	Mar. 05, 2015 USD ($) shares	Sep. 30, 2019 USD ($)	Mar. 31, 2018 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Sep. 30, 2019 £ / shares shares	Dec. 31, 2018 £ / shares shares	Dec. 31, 2015 shares	Sep. 30, 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 | $					$ 440,108,000	$ 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)	22,222,223	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