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EMPLOYEE BENEFIT PLANS - PENSION AND OTHER POSTEMPLOYMENT BENEFIT PLANS, NARRATIVE (Details) | 12 Months Ended
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Sep. 30, 2020 USD ($) plan | Sep. 30, 2019 USD ($) | Sep. 30, 2018 USD ($)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Number of noncontributory defined benefit retirement plans (in plans) | plan | 2 |  |
Required number of years of service (more than) | 1 year |  |
Years of service and average compensation, basis period for plan benefits | 60 months |  |
Pension |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Employer discretionary contributions | $ 0 | $ 0 | $ 0
Employer contributions | 596,000 | 231,000 |
OPEB |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Employer contributions | 8,400,000 | $ 7,926,000 | 6,200,000
OPEB | Minimum |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Estimated future employer contributions over the next five years | 5,000,000 |  |
OPEB | Maximum |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Estimated future employer contributions over the next five years | $ 10,000,000 |  |
Special Termination Benefits | Voluntary Early Retirement Program |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Pension and other postretirement benefits cost |  |  | 4,200,000
Other Severance Benefits | Voluntary Early Retirement Program |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Pension and other postretirement benefits cost |  |  | 2,200,000
Home Services and Other | Voluntary Early Retirement Program |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Pension and other postretirement benefits cost |  |  | 1,300,000
NJNG | Voluntary Early Retirement Program |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Pension and other postretirement benefits cost |  |  | $ 5,100,000

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