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Income Taxes - Additional Information (Detail) - USD ($) | 12 Months Ended
--- | ---
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 2019 | $ 0 |  |
Change in corporate tax rate | 21.00% | 34.00% |
Non-cash tax expense recognized related to recording of additional valuation allowance | $ 140,117,000 | $ 131,389,000 | $ 109,274,000
Valuation allowance to reduce deferred tax assets |  | 0 |
Non-cash tax charge due to decrease in corporate tax rate | 6,736,000 | 5,788,000 | 4,524,000
Decrease in net operating loss carryforwards from prior years | 78,300,000 |  |
Tax credit carryforwards | 7,816,000 | 6,378,000 |
Undistributed earning retained indefinitely for reinvestment by foreign subsidiary | 8,400,000 |  |
ASU No. 2016-09 |  |  |
Income Taxes [Line Items] |  |  |
Provisions benefit, (expense) for income taxes | $ 7,700,000 | $ 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 |
U.S., Republic of Ireland, United Kingdom and Israel |  |  |
Income Taxes [Line Items] |  |  |
Foreign net operating loss carryforwards | $ 788,700,000 |  |
U.S. Federal |  |  |
Income Taxes [Line Items] |  |  |
Tax credit carryforwards | $ 7,100,000 |  |
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 | 2038 |  |
State |  |  |
Income Taxes [Line Items] |  |  |
Tax credit carryforwards | $ 2,100,000 |  |
State | Minimum |  |  |
Income Taxes [Line Items] |  |  |
Tax credit carryforward expiration year | 2024 |  |
State | Maximum |  |  |
Income Taxes [Line Items] |  |  |
Tax credit carryforward expiration year | 2038 |  |

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