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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 |  |

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