Inventory - Schedule of Inventories (Parenthetical) (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Inventory Disclosure [Abstract]
Finished goods inventory located at third-party warehouse and shipping service provider	$ 7.6	$ 11.0

Carrying amount, net of valuation reserves and adjustments, as of the balance sheet date of merchandise or goods held by the company that are readily available for sale, located at third party warehouse and shipping service provider.

No definition available.

Name:	alks_InventoryFinishedGoodsNetOfReservesLocatedAtThirdPartyWarehouseAndShippingServiceProvider
Namespace Prefix:	alks_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant