FAIR VALUE - DEBT (Details) - Significant Other Observable Inputs (Level 2) - USD ($) $ in Thousands	Sep. 30, 2019	Sep. 30, 2018
NJNG | Carrying value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, fair value	$ 892,845	$ 672,045
NJNG | Fair market value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, fair value	984,129	669,162
NJR | Carrying value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, fair value	550,000	500,000
NJR | Fair market value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, fair value	$ 584,735	$ 488,889