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Deferred Tax Liabilities, Subject to Expiration

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Employee service share based compensation tax benefit (deficiency) realized from exercise of stock options.

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Income tax.

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Statutory Tax Rate

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Tax credit carryforward expiration year.

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Amount after allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences and carryforwards classified as noncurrent.

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible foreign operating loss carryforwards.

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Amount, before allocation of a valuation allowances, of deferred tax assets attributable to deductible tax credit carryforwards including, but not limited to, research, foreign, general business, alternative minimum tax, and other deductible tax credit carryforwards.

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Amount of deferred tax assets for which it is more likely than not that a tax benefit will not be realized.

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Percentage of domestic federal statutory tax rate applicable to pretax income (loss).

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Amount after tax of cumulative effect of restatement on the opening retained earnings of the earliest year presented.

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Amount of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations, attributable to increase (decrease) in the income tax rates.

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Amount of undistributed earnings of foreign subsidiaries intended to be permanently reinvested outside the country of domicile.

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The total amount of unrecognized tax benefits that, if recognized, would affect the effective tax rate.

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Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Taxes [Line Items]
Total amount of unrecognized tax benefits that would affect the tax rate if recognized		$ 0	$ 1,500,000
Gross liabilities to expire in 2018		$ 0
Change in corporate tax rate		34.00%
Non-cash tax expense recognized related to recording of additional valuation allowance		$ 131,389,000	109,274,000	$ 95,999,000
Valuation allowance to reduce deferred tax assets		0	11,082,000
Non-cash tax charge due to decrease in corporate tax rate		5,788,000	4,524,000	$ 2,248,000
Decrease in net operating loss carryforwards from prior years		109,200,000
Tax credit carryforwards		6,378,000	6,893,000
Undistributed earning retained indefinitely for reinvestment by foreign subsidiary		8,000,000
Scenario, Forecast
Income Taxes [Line Items]
Change in corporate tax rate	21.00%
ASU No. 2016-09
Income Taxes [Line Items]
Provisions benefit, (expense) for income taxes		$ 1,300,000	(400,000)
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
Republic of Ireland, United Kingdom and Israel
Income Taxes [Line Items]
Foreign net operating loss carryforwards		710,400,000
U.S. Federal
Income Taxes [Line Items]
Tax credit carryforwards		$ 6,000,000
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		2036
State
Income Taxes [Line Items]
Tax credit carryforwards		$ 1,900,000
State \| Minimum
Income Taxes [Line Items]
Tax credit carryforward expiration year		2024
State \| Maximum
Income Taxes [Line Items]
Tax credit carryforward expiration year		2036