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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - SOFTWARE COSTS (Details) - Software, In-Development, Internally Developed, and Purchased - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025	Sep. 30, 2025
Oil and Gas, Full Cost Method, Capitalized Cost Excluded from Amortization [Line Items]
Capitalized software costs	$ 11,037		$ 11,037		$ 11,151
Location, Statement of Income, Balance [Axis]: us-gaap:DepreciationDepletionAndAmortization
Oil and Gas, Full Cost Method, Capitalized Cost Excluded from Amortization [Line Items]
Software costs	5,601	$ 3,190	12,128	$ 8,733
Location, Statement of Income, Balance [Axis]: us-gaap:OtherCostAndExpenseOperating
Oil and Gas, Full Cost Method, Capitalized Cost Excluded from Amortization [Line Items]
Software costs	182	$ 1,335	698	$ 5,221
Location, Statement of Financial Position, Balance [Axis]: us-gaap:PropertyPlantAndEquipmentOther
Oil and Gas, Full Cost Method, Capitalized Cost Excluded from Amortization [Line Items]
Software costs	344		344		344
Location, Statement of Financial Position, Balance [Axis]: us-gaap:PublicUtilitiesPropertyPlantAndEquipmentAccumulatedDepreciation
Oil and Gas, Full Cost Method, Capitalized Cost Excluded from Amortization [Line Items]
Accumulated depreciation and amortization	(37,011)		(37,011)		(24,906)
Location, Statement of Financial Position, Balance [Axis]: us-gaap:PublicUtilitiesPropertyPlantAndEquipmentPlantInService
Oil and Gas, Full Cost Method, Capitalized Cost Excluded from Amortization [Line Items]
Software costs	254,028		254,028		132,868
Location, Statement of Financial Position, Balance [Axis]: njr:AccumulatedDepreciationDepletionandAmortizationOtherPropertyPlantandEquipment
Oil and Gas, Full Cost Method, Capitalized Cost Excluded from Amortization [Line Items]
Accumulated depreciation and amortization	(93)		(93)		(70)
Location, Statement of Financial Position, Balance [Axis]: njr:PublicUtilitiesPropertyPlantandEquipmentWorkinProgress \| Regulated
Oil and Gas, Full Cost Method, Capitalized Cost Excluded from Amortization [Line Items]
Software costs	$ 0		$ 0		$ 87,274