v3.25.4

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Investments in Marketable Securities and Cash Equivalents - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2025
Accounting Policies [Abstract]
Marketable securities classified as cash equivalents, maximum original maturity (in days)	90 days
Policy for marketable securities (in days)	90 days

X

- Definition

Purchases are classified as marketable securities if their original maturity, from the date of purchase, is in excess of 90 days.

+ References

No definition available.

+ Details

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X

- Definition

Purchases are classified as cash equivalents if their original maturity, from the date of purchase, is 90 days or less.

+ References

No definition available.

+ Details

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Namespace Prefix:	alny_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

X

- References

No definition available.

+ Details

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