Leases - Additional Information (Detail) $ in Thousands						3 Months Ended	9 Months Ended
	Aug. 15, 2019 USD ($) RenewalOption	Jul. 04, 2019	Apr. 12, 2019	Jan. 26, 2019	Jul. 01, 2011	Sep. 30, 2019 USD ($)	Sep. 30, 2019 USD ($)
Lessee Lease Description [Line Items]
Incremental borrowing rate for purposes of calculation of lease liabilities						11.50%	11.50%
Change in incremental borrwing rate							1.00%
Monthly rent payment						$ 18,223	$ 18,223
Operating lease liability						9,556	9,556
Operating lease right-of-use asset						8,633	8,633
Operating lease expense						$ 400	$ 1,100
Dublin | Office Centre Sharing Agreements
Lessee Lease Description [Line Items]
Description of lease agreement							On April 12, 2019 and July 4, 2019, the Company entered into Office Centre Sharing Agreements effective May 1, 2019 and October 1, 2019, respectively, to replace the office space with new office space in Dublin, Ireland which terminates on April 30, 2020 and September 30, 2020, respectively, and can be extended automatically for successive one year periods.
Lease termination existence of option to extend							true
Lease renewal term						1 year	1 year
Operating lease, option to extend							can be extended automatically for successive one year periods.
Operating lease, existence of option to extend [true false]							true
Dublin | Office Centre Sharing Agreements Effective May 1, 2019
Lessee Lease Description [Line Items]
Operating lease, expiration date			Apr. 30, 2020
Dublin | Office Centre Sharing Agreements Effective October 1, 2019
Lessee Lease Description [Line Items]
Operating lease, expiration date		Sep. 30, 2020
Bedminster
Lessee Lease Description [Line Items]
Operating lease, expiration date				Sep. 15, 2019	Sep. 15, 2019
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 termination existence of option to extend	true
Lease renewal term	5 years
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