Equity - Preferred Stock - Additional Information (Detail)		3 Months Ended	6 Months Ended	12 Months Ended
	Mar. 05, 2015 USD ($) shares	Mar. 31, 2015 USD ($)	Jun. 30, 2021 £ / shares shares	Dec. 31, 2020 £ / shares shares	Dec. 31, 2018 shares	Dec. 31, 2015 shares	Mar. 05, 2015 £ / shares	Mar. 05, 2015 $ / shares
Stockholders Equity Note [Line Items]
Preferred stock, par value | £ / shares							£ 0.05
Estimated offering expense | $		$ 700,000
Common stock, par value | £ / shares			£ 0.50	£ 0.50			£ 0.50
Series A Preferred Stock
Stockholders Equity Note [Line Items]
American depositary shares conversion rate to preference shares			American Depositary Shares, or ADSs, each representing one share of Amarin’s Series A Convertible Preference Shares
Ordinary Shares
Stockholders Equity Note [Line Items]
Convertible senior notes, total ADS into which the debt is exchangeable				28,931,746	3,886,718	6,283,333
Preferred stock, equivalent ordinary shares upon future consolidation outstanding			0	0
American Depositary Shares | Series A Preferred Stock
Stockholders Equity Note [Line Items]
Issuance of stock (shares)	391,017,970
Stock issued under private placement, value | $	$ 58,600,000
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