FAIR VALUE - DEBT (Details) - Significant Other Observable Inputs (Level 2) - USD ($) $ in Thousands		Sep. 30, 2018	Sep. 30, 2017
Natural Gas Distribution | Carrying value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, fair value	[1]	$ 672,045	$ 672,045
Natural Gas Distribution | Fair market value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, fair value	[1]	669,162	673,051
NJR | Carrying value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, fair value	[1]	500,000	425,000
NJR | Fair market value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, fair value	[1]	$ 488,889	$ 434,625

[1] See Note 8. Debt for a reconciliation to long-term and short-term debt.