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Income tax.

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Statutory Tax Rate

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Income Taxes - Additional Information (Detail) - USD ($)		12 Months Ended
	Mar. 27, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Taxes [Line Items]
Total amount of unrecognized tax benefits that would affect the tax rate if recognized		$ 5,600,000	$ 0
Gross liabilities to expire in 2021		0
Provision for income taxes		(745,000)	(164,000)	$ (96,000)
Increase in net operating loss carryforwards from prior years		38,700,000
Tax credit carryforwards		14,690,000	9,149,000
Undistributed earning retained indefinitely for reinvestment by foreign subsidiary		0
Unrecognized deferred income tax liability		0
ASU No. 2016-09
Income Taxes [Line Items]
Provisions benefit for income taxes		3,700,000	21,900,000	7,700,000
CARES Act
Income Taxes [Line Items]
Carry back net operating losses arising beginning year	2018
Carry back net operating losses arising end year	2020
Net operating loss carryback period	5 years
Provision for income taxes		$ 2,500,000	$ 0	$ 0
Ireland
Income Taxes [Line Items]
Corporate tax rate		25.00%	25.00%	25.00%
Ireland \| Amarin Pharmaceuticals Ireland Limited
Income Taxes [Line Items]
Corporate tax rate		12.50%	12.50%	12.50%
Ireland \| Non-Trading Activity
Income Taxes [Line Items]
Corporate tax rate		25.00%
Ireland \| Trading Activity
Income Taxes [Line Items]
Corporate tax rate		12.50%
U.S., Republic of Ireland, United Kingdom and Israel
Income Taxes [Line Items]
Foreign net operating loss carryforwards		$ 900,500,000
U.S. Federal
Income Taxes [Line Items]
Tax credit carryforwards		$ 13,000,000.0
U.S. Federal \| Minimum
Income Taxes [Line Items]
Tax credit carryforward expiration year		2024
U.S. Federal \| Maximum
Income Taxes [Line Items]
Tax credit carryforward expiration year		2040
State
Income Taxes [Line Items]
Tax credit carryforwards		$ 4,000,000.0
State \| Minimum
Income Taxes [Line Items]
Tax credit carryforward expiration year		2024
State \| Maximum
Income Taxes [Line Items]
Tax credit carryforward expiration year		2040