Long-term Debt - Additional Information (Details) $ in Millions	6 Months Ended	12 Months Ended
Jun. 30, 2021 USD ($) Lender	Dec. 31, 2020
Long-term debt
Number of lender holding percentage of outstanding amount \| Lender	3
Interest Expense
Long-term debt
Refinancing charges	$ 2.1
Minimum
Long-term debt
Threshold percentage comparing present value of remaining cash flows	10.00%
Percentage of outstanding principal amount held by lender	2.00%
Maximum
Long-term debt
Percentage of outstanding principal amount held by lender	7.00%
2026 Term Loans
Long-term debt
Due date of loan	Mar. 12, 2026
Term Loans	$ 300.0
Variable interest rate base	LIBOR
Incremental capacity	$ 175.0
2026 Term Loans \| LIBOR
Long-term debt
Interest rate added to base rate (as a percent)	2.50%
Interest rate, variable interest rate floor (as a percent)	0.50%
2023 Term Loans
Long-term debt
Due date of loan		Mar. 26, 2023
Variable interest rate base		LIBOR
2023 Term Loans \| LIBOR
Long-term debt
Interest rate added to base rate (as a percent)		2.25%
Interest rate, variable interest rate floor (as a percent)		0.00%

Represents the amount of incremental capacity available so long as the entity meets certain conditions.

No definition available.

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Namespace Prefix:	alks_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The floor for the variable rate base of the debt instrument.

+ References

No definition available.

+ Details

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- Definition

Represents the number of lender holding percentage of outstanding amount.

+ References

No definition available.

+ Details

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- Definition

Percentage of outstanding principal amount held by lender.

+ References

No definition available.

+ Details

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- Definition

Threshold percentage of present value of remaining cash flow difference used for determining if debt terms substantially different.

+ References

No definition available.

+ Details

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- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

No definition available.

+ Details

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Balance Type:	na
Period Type:	duration

- Definition

The reference rate for the variable rate of the debt instrument, such as LIBOR or the US Treasury rate and the maturity of the reference rate used, such as three months or six months LIBOR.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
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- Definition

Face (par) amount of debt instrument at time of issuance.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=119993939&loc=d3e28551-108399

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-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=114775985&loc=d3e28878-108400

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentLineItems
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Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(2))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

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-Publisher FASB
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-Section 50
-Paragraph 2
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+ Details

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- Definition

Amount of a short-term obligation that has been excluded from current liabilities in the balance sheet because of a refinancing of the obligation.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=6802200&loc=d3e1927-112601

+ Details

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- Details

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- Details

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