FAIR VALUE - DEBT (Details) - USD ($) $ in Thousands	Dec. 31, 2024	Sep. 30, 2024
NJR
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt issuance costs	$ 3,400	$ 3,000
NJNG
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt issuance costs	10,800	10,900
Level 2 | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, fair value	2,767,845	2,767,845
Level 2 | Fair market value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, fair value	$ 2,391,501	$ 2,525,804