INCOME TAXES - Schedule of Domestic and Foreign Components of Income (Loss) before Income Taxes (Detail) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended	9 Months Ended	12 Months Ended
	Mar. 31, 2025	Jun. 30, 2025	Sep. 30, 2025	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Income Tax Disclosure [Abstract]
Domestic				$ 18,300	$ (571,926)	$ (450,311)
Foreign				304,852	194,551	16,794
Income (loss) before income taxes	$ (2,368)	$ (43,677)	$ 236,891	$ 323,152	$ (377,375)	$ (433,517)

Amount of income (loss) from continuing operations, including income (loss) from equity method investments, before deduction of income tax expense (benefit), and income (loss) attributable to noncontrolling interest.

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ALNYLAM PHARMACEUTICALS, INC. 10-K