COMMITMENTS AND CONTINGENT LIABILITIES - CAPITAL EXPENDITURES (Details) $ in Millions	Sep. 30, 2021 USD ($)
Commitments and Contingencies Disclosure [Abstract]
Operating leases, future minimum payments due, next five years (not more than)	$ 8.0

Lessee, Operating Lease, Liability, Payments, Due Next Five Years

No definition available.

Name:	njr_LesseeOperatingLeaseLiabilityPaymentsDueNextFiveYears
Namespace Prefix:	njr_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant