Leases - Additional Information (Detail) $ in Thousands				3 Months Ended
	Aug. 15, 2019 USD ($) RenewalOption	Jul. 04, 2019	Apr. 12, 2019	Mar. 31, 2020 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2019 USD ($)
Lessee Lease Description [Line Items]
Incremental borrowing rate for purposes of calculation of lease liabilities				11.50%
Change in incremental borrowing rate				1.00%
Short-term lease payments				$ 100
Monthly rent payment				18,223
Operating lease liability				10,100		$ 9,800
Operating lease right-of-use asset				8,397		$ 8,511
Operating lease expense				$ 400	$ 300
Dublin | Office Centre Sharing Agreements
Lessee Lease Description [Line Items]
Description of lease agreement				On April 12, 2019 the Company entered into an Office Centre Sharing Agreement for office space in Dublin, Ireland effective May 1, 2019 was scheduled to terminate on April 30, 2020 and was extended for one additional year through April 30, 2021 and can continue to be extended automatically for successive one year periods. On July 4, 2019, the Company entered into an Office Centre Sharing Agreements effective October 1, 2019 for office space in Dublin, Ireland which terminates on September 30, 2020 and can be extended automatically for successive one year periods.
Operating lease, expiration date		Sep. 30, 2020	Apr. 30, 2020
Lease renewal term				1 year
Operating lease, option to additional extend				extended for one additional year through April 30, 2021 and can continue to be extended automatically for successive one year periods
Lease termination existence of option to extend				true
Operating lease, existence of option to extend [true false]				true
Operating lease, option to extend				On July 4, 2019, the Company entered into an Office Centre Sharing Agreements effective October 1, 2019 for office space in Dublin, Ireland which terminates on September 30, 2020 and can be extended automatically for successive one year periods.
Dublin | Office Centre Sharing Agreements Effective May 1, 2019
Lessee Lease Description [Line Items]
Operating lease, expiration date			Apr. 30, 2021
Bridgewater
Lessee Lease Description [Line Items]
Description of lease agreement				Amarin will have a one-time option to terminate the agreement effective on the first day of the ninety-seventh month after the Commencement Date upon advance written notice and a termination payment specified in the Lease.
Lease renewal term	5 years
Lease termination existence of option to extend	true
Lease commencement date	Aug. 15, 2019
Lease agreement term	11 years
Number of renewal options | RenewalOption	2
Lease termination description				one-time option to terminate the agreement effective on the first day of the ninety-seventh month after the Commencement Date
Monthly rent payment	$ 100
Maximum
Lessee Lease Description [Line Items]
Short term lease period				12 months