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Income tax.

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Income Taxes - Additional Information (Detail) - USD ($)		12 Months Ended
	Mar. 27, 2020	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Income Taxes [Line Items]
Total amount of unrecognized tax benefits that would affect the tax rate if recognized		$ 7,900,000	$ 5,600,000
Interest related to uncertain tax positions		900,000	0
Penalties related to uncertain tax positions		0	0
Gross liabilities to expire in 2022		0
Provision for income taxes		(3,562,000)	(745,000)	$ (164,000)
Decrease in net operating loss carryforwards from prior years		50,600,000
Tax credit carryforwards		15,803,000	14,690,000
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		100,000	3,700,000	21,900,000
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		$ 0	$ 2,500,000	$ 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		$ 849,900,000
U.S. Federal
Income Taxes [Line Items]
Tax credit carryforwards		$ 13,700,000
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		$ 4,500,000
State \| Minimum
Income Taxes [Line Items]
Tax credit carryforward expiration year		2024
State \| Maximum
Income Taxes [Line Items]
Tax credit carryforward expiration year		2040