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Co-Promotion Agreement - Additional Information (Detail) - USD ($) | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2019 | Dec. 31, 2018
Co Promotion Agreement [Line Items] |  |
Accrued expenses and other current liabilities | $ 94,170,000 | $ 84,171,000
Co-Promotion Agreement (the Agreement) | Kowa Pharmaceuticals America |  |
Co Promotion Agreement [Line Items] |  |
Agreement amended date | Jul. 25, 2017 |
Promotion fee as a percentage of gross margin |  | 18.50%
Maximum co-promotion tail payments receive period | 3 years |
Co-promotion tail payments receivable |  | $ 17,800,000
Tail payments during the period | $ 0 |
Accrued co- promotion tail payments | 16,900,000 | $ 16,600,000
Accrued expenses and other current liabilities | $ 9,500,000 |

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- Definition Accrued co promotion tail payments. + References No definition available. + Details
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Namespace Prefix: | amrn_
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Balance Type: | credit
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- Definition Accrued liabilities and other liabilities current. + References No definition available. + Details
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Balance Type: | credit
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- Definition Agreement amended date. + References No definition available. + Details
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- Definition Co-promotion agreement. + References No definition available. + Details
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- Definition Co-Promotion tail payment. + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Co-Promotion tail payments receivable. + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | debit
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- Definition Maximum co-promotion tail payments receive period. + References No definition available. + Details
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- Definition Percentage of gross margin, payable for promotion agreement. + References No definition available. + Details
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Namespace Prefix: | amrn_
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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | srt_CounterpartyNameAxis=amrn_KowaPharmaceuticalsAmericaMember
Namespace Prefix: |
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Balance Type: |
Period Type: |