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Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Taxes [Line Items]
Total amount of unrecognized tax benefits that would affect the tax rate if recognized	$ 0	$ 0
Gross liabilities to expire in 2020	$ 0
Change in corporate tax rate	21.00%	21.00%	34.00%
Non-cash tax expense recognized related to recording of additional valuation allowance	$ 137,976,000	$ 140,117,000	$ 131,389,000
Valuation allowance to reduce deferred tax assets			0
Non-cash tax charge due to decrease in corporate tax rate	(3,222,000)	6,736,000	5,788,000
Increase in net operating loss carryforwards from prior years	73,100,000
Tax credit carryforwards	9,149,000	7,816,000
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	$ 21,900,000	$ 7,700,000	$ 1,300,000
Cumulative-effect adjustment to accumulated deficit				$ 1,600,000
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%
Tax Cuts and Jobs Act
Income Taxes [Line Items]
Non-cash tax expense recognized related to recording of additional valuation allowance			$ 8,700,000
Non-cash tax charge due to decrease in corporate tax rate			$ 2,400,000
U.S., Republic of Ireland, United Kingdom and Israel
Income Taxes [Line Items]
Foreign net operating loss carryforwards	$ 861,800,000
U.S. Federal
Income Taxes [Line Items]
Tax credit carryforwards	$ 8,400,000
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	2039
State
Income Taxes [Line Items]
Tax credit carryforwards	$ 2,400,000
State | Minimum
Income Taxes [Line Items]
Tax credit carryforward expiration year	2024
State | Maximum
Income Taxes [Line Items]
Tax credit carryforward expiration year	2039

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