FAIR VALUE - DEBT (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Sep. 30, 2023
NJR
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt issuance costs	$ 3,300	$ 3,700
NJNG
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt issuance costs	10,000	9,800
Level 2 | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, fair value	2,567,845	2,587,845
Level 2 | Fair market value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, fair value	$ 2,218,977	$ 2,106,536