Nature of Business and Basis of Presentation	12 Months Ended
Dec. 31, 2017
Accounting Policies [Abstract]
Nature of Business and Basis of Presentation	(1) Nature of Business and Basis of Presentation Nature of Business Amarin Corporation plc (“Amarin” or the “Company”) is a biopharmaceutical company with expertise in lipid science focused on the commercialization and development of therapeutics to improve cardiovascular health. The Company’s lead product, Vascepa^® (icosapent ethyl) capsules, is approved by the U.S. Food and Drug Administration, or FDA, for use as an adjunct to diet to reduce triglyceride levels in adult patients with severe (TG ≥500 mg/dL) hypertriglyceridemia. Vascepa is available in the United States by prescription only. In January 2013, the Company began selling and marketing 1-gram size Vascepa capsules in the United States, and in October 2016, introduced a smaller 0.5-gram capsule size. In August 2015, in addition to marketing Vascepa for severe hypertriglyceridemia, the Company commenced marketing Vascepa for use in adult patients with mixed dyslipidemia, as an adjunct to diet and an add-on to statin therapy in patients who despite statin therapy have high triglycerides (TGs ≥200 mg/dL and ≤500 mg/dL), which the Company also refers to as persistently high triglycerides. This expanded promotion of Vascepa commenced pursuant to a federal court order and is continuing pursuant to an agreement among the Company, the FDA and the U.S. government. The Company is also developing Vascepa for FDA approval of potential additional indications for use. In particular, the Company is conducting a cardiovascular outcomes study of Vascepa, titled REDUCE-IT (Reduction of Cardiovascular Events with EPA—Intervention Trial). The REDUCE-IT study, which commenced in 2011 and completed patient enrollment and randomization of 8,175 individual patients in 2016, is designed to evaluate the efficacy of Vascepa in reducing major cardiovascular events in a high-risk patient population on statin therapy. The Company anticipates that results of the REDUCE-IT study will be available and made public before the end of the third quarter of 2018. The Company sells Vascepa principally to a limited number of major wholesalers, as well as selected regional wholesalers and specialty pharmacy providers, or collectively, its Distributors or its customers, that in turn resell Vascepa to retail pharmacies for subsequent resale to patients and healthcare providers. The Company markets Vascepa through its direct sales force of approximately 165 sales professionals, including sales representatives and their managers, and through a co-promotion agreement with Kowa Pharmaceuticals America, Inc. Under this co-promotion agreement, which commenced in May 2014 and is scheduled to end at the end of 2018, Kowa Pharmaceuticals America, Inc. co-promotes Vascepa in conjunction with its promotion of its primary product, a branded statin for patients with high cholesterol. The Company operates in one business segment. Basis of Presentation The consolidated financial statements included herein have been prepared by the Company in accordance with accounting principles generally accepted in the United States of America (the “U.S.” or the “United States”) and pursuant to the rules and regulations of the Securities and Exchange Commission, or the SEC. The consolidated financial statements reflect all adjustments of a normal and recurring nature that, in the opinion of management, are necessary to present fairly the Company’s financial position, results of operations and cash flows for the periods indicated. The preparation of the Company’s consolidated financial statements in conformity with U.S. Generally Accepted Accounting Principles (“GAAP”) requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, disclosure of contingent assets and liabilities at the date of the financial statements, and the reported amounts of revenues and expenses during the reporting period. The results of operations for the years ended December 31, 2017, 2016 and 2015 are not necessarily indicative of the results for any future period. Certain numbers presented throughout this document may not add precisely to the totals provided due to rounding. Absolute and percentage changes are calculated using the underlying amounts in thousands. Certain prior year balances related to deferred tax assets and liabilities and the provision for income taxes have been reclassified to conform to the current year presentation. These reclassifications do not have a material impact on the Company’s consolidated financial statements. The accompanying consolidated financial statements of the Company and subsidiaries have been prepared on a basis which assumes that the Company will continue as a going concern, which contemplates the realization of assets and the satisfaction of liabilities and commitments in the normal course of business. At December 31, 2017, the Company had current assets of $153.3 million, including cash and cash equivalents of $73.6 million. The Company’s consolidated balance sheets also include long-term debt from a royalty-bearing instrument and exchangeable senior notes. In January 2017, the Company issued $30.0 million in aggregate principal amount of January 2017 3.5% exchangeable senior notes due 2047, or the 2017 Notes. The terms of the 2017 Notes are such that they may be redeemed by the Company for cash on or after January 19, 2021 and may be put back to the Company by the holders on January 19, 2022 for cash equal to 100% of the principal amount plus any accrued and unpaid interest. The 2017 Notes are exchangeable into American Depositary Shares (“ADSs”) at the option of holders at any time after issuance and prior to maturity and are exchangeable into ADSs at the option of the Company upon satisfaction of certain equity conditions. Accordingly, the exchangeable senior notes do not represent a short-term claim on the liquid assets of the Company as of December 31, 2017. The terms of the Company’s January 2012 3.5% exchangeable senior notes due 2032, or the 2012 Notes, which were repaid in full during the first quarter of 2017, allowed for repurchase in cash by the Company at the option of the holders on January 19, 2017, as well as redemption by the Company for cash of all or part of the 2012 Notes on or after January 19, 2017, both at a price equal to 100% of the principal amount of the 2012 Notes to be repurchased or redeemed, plus accrued and unpaid interest to, but excluding, the repurchase or redemption date. Accordingly, $15.1 million in principal amount of 2012 Notes represented a short-term claim on the liquid assets of the Company as of December 31, 2016. Refer to Note 8—Debt for further discussion. In February 2018, the Company received approximately $65.0 million of net proceeds from a registered offering of its ADSs (refer to Note 18—Subsequent Events for further discussion). The Company believes its cash and cash equivalents will be sufficient to fund its projected operations through the results of the REDUCE-IT study, which we anticipate will be available before the end of the third quarter of 2018 and, assuming positive results of the REDUCE-IT study, through subsequent public presentation of such results at a medical congress before the end of 2018. Depending on the level of cash generated from operations, additional capital may be required to expand promotion of Vascepa as contemplated following anticipated successful results of the REDUCE-IT study. If additional capital is required and the Company is unable to obtain additional capital, the Company may be forced to delay, limit or eliminate all or a portion of the expanded promotional activities planned following successful results of the REDUCE-IT study. The Company anticipates that annual net cash outflows in future periods will be variable.

(1) Nature of Business and Basis of Presentation

Nature of Business

Amarin Corporation plc (“Amarin” or the “Company”) is a biopharmaceutical company with expertise in lipid science focused on the commercialization and development of therapeutics to improve cardiovascular health.

The Company’s lead product, Vascepa^® (icosapent ethyl) capsules, is approved by the U.S. Food and Drug Administration, or FDA, for use as an adjunct to diet to reduce triglyceride levels in adult patients with severe (TG ≥500 mg/dL) hypertriglyceridemia. Vascepa is available in the United States by prescription only. In January 2013, the Company began selling and marketing 1-gram size Vascepa capsules in the United States, and in October 2016, introduced a smaller 0.5-gram capsule size. In August 2015, in addition to marketing Vascepa for severe hypertriglyceridemia, the Company commenced marketing Vascepa for use in adult patients with mixed dyslipidemia, as an adjunct to diet and an add-on to statin therapy in patients who despite statin therapy have high triglycerides (TGs ≥200 mg/dL and ≤500 mg/dL), which the Company also refers to as persistently high triglycerides. This expanded promotion of Vascepa commenced pursuant to a federal court order and is continuing pursuant to an agreement among the Company, the FDA and the U.S. government.

The Company is also developing Vascepa for FDA approval of potential additional indications for use. In particular, the Company is conducting a cardiovascular outcomes study of Vascepa, titled REDUCE-IT (Reduction of Cardiovascular Events with EPA—Intervention Trial). The REDUCE-IT study, which commenced in 2011 and completed patient enrollment and randomization of 8,175 individual patients in 2016, is designed to evaluate the efficacy of Vascepa in reducing major cardiovascular events in a high-risk patient population on statin therapy. The Company anticipates that results of the REDUCE-IT study will be available and made public before the end of the third quarter of 2018.

The Company sells Vascepa principally to a limited number of major wholesalers, as well as selected regional wholesalers and specialty pharmacy providers, or collectively, its Distributors or its customers, that in turn resell Vascepa to retail pharmacies for subsequent resale to patients and healthcare providers. The Company markets Vascepa through its direct sales force of approximately 165 sales professionals, including sales representatives and their managers, and through a co-promotion agreement with Kowa Pharmaceuticals America, Inc. Under this co-promotion agreement, which commenced in May 2014 and is scheduled to end at the end of 2018, Kowa Pharmaceuticals America, Inc. co-promotes Vascepa in conjunction with its promotion of its primary product, a branded statin for patients with high cholesterol. The Company operates in one business segment.

Basis of Presentation

The consolidated financial statements included herein have been prepared by the Company in accordance with accounting principles generally accepted in the United States of America (the “U.S.” or the “United States”) and pursuant to the rules and regulations of the Securities and Exchange Commission, or the SEC.

The consolidated financial statements reflect all adjustments of a normal and recurring nature that, in the opinion of management, are necessary to present fairly the Company’s financial position, results of operations and cash flows for the periods indicated. The preparation of the Company’s consolidated financial statements in conformity with U.S. Generally Accepted Accounting Principles (“GAAP”) requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, disclosure of contingent assets and liabilities at the date of the financial statements, and the reported amounts of revenues and expenses during the reporting period. The results of operations for the years ended December 31, 2017, 2016 and 2015 are not necessarily indicative of the results for any future period. Certain numbers presented throughout this document may not add precisely to the totals provided due to rounding. Absolute and percentage changes are calculated using the underlying amounts in thousands. Certain prior year balances related to deferred tax assets and liabilities and the provision for income taxes have been reclassified to conform to the current year presentation. These reclassifications do not have a material impact on the Company’s consolidated financial statements.

The accompanying consolidated financial statements of the Company and subsidiaries have been prepared on a basis which assumes that the Company will continue as a going concern, which contemplates the realization of assets and the satisfaction of liabilities and commitments in the normal course of business.

At December 31, 2017, the Company had current assets of $153.3 million, including cash and cash equivalents of $73.6 million. The Company’s consolidated balance sheets also include long-term debt from a royalty-bearing instrument and exchangeable senior notes. In January 2017, the Company issued $30.0 million in aggregate principal amount of January 2017 3.5% exchangeable senior notes due 2047, or the 2017 Notes. The terms of the 2017 Notes are such that they may be redeemed by the Company for cash on or after January 19, 2021 and may be put back to the Company by the holders on January 19, 2022 for cash equal to 100% of the principal amount plus any accrued and unpaid interest. The 2017 Notes are exchangeable into American Depositary Shares (“ADSs”) at the option of holders at any time after issuance and prior to maturity and are exchangeable into ADSs at the option of the Company upon satisfaction of certain equity conditions. Accordingly, the exchangeable senior notes do not represent a short-term claim on the liquid assets of the Company as of December 31, 2017. The terms of the Company’s January 2012 3.5% exchangeable senior notes due 2032, or the 2012 Notes, which were repaid in full during the first quarter of 2017, allowed for repurchase in cash by the Company at the option of the holders on January 19, 2017, as well as redemption by the Company for cash of all or part of the 2012 Notes on or after January 19, 2017, both at a price equal to 100% of the principal amount of the 2012 Notes to be repurchased or redeemed, plus accrued and unpaid interest to, but excluding, the repurchase or redemption date. Accordingly, $15.1 million in principal amount of 2012 Notes represented a short-term claim on the liquid assets of the Company as of December 31, 2016. Refer to Note 8—Debt for further discussion.

In February 2018, the Company received approximately $65.0 million of net proceeds from a registered offering of its ADSs (refer to Note 18—Subsequent Events for further discussion). The Company believes its cash and cash equivalents will be sufficient to fund its projected operations through the results of the REDUCE-IT study, which we anticipate will be available before the end of the third quarter of 2018 and, assuming positive results of the REDUCE-IT study, through subsequent public presentation of such results at a medical congress before the end of 2018. Depending on the level of cash generated from operations, additional capital may be required to expand promotion of Vascepa as contemplated following anticipated successful results of the REDUCE-IT study. If additional capital is required and the Company is unable to obtain additional capital, the Company may be forced to delay, limit or eliminate all or a portion of the expanded promotional activities planned following successful results of the REDUCE-IT study. The Company anticipates that annual net cash outflows in future periods will be variable.