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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - ADDITIONAL INFORMATION (Details) shares in Thousands |  |  |  |  | 12 Months Ended |  |
--- | --- | --- | --- | --- | --- | --- | ---
Jul. 01, 2020 | Jan. 13, 2020 mi | Mar. 06, 2019 USD ($) | Jan. 28, 2019 shares | Sep. 30, 2020 USD ($) plan | Sep. 30, 2019 USD ($) | Sep. 30, 2018 USD ($) | Oct. 11, 2019 USD ($) | Oct. 01, 2019 USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |
Composite rate of depreciation |  |  |  |  | 2.65% | 2.25% | 2.29% |  |
Depreciation and amortization |  |  |  |  | $ 107,368,000 | $ 81,109,000 | $ 76,571,000 |  |
SBC interest rate |  |  |  |  | 1.97% | 3.30% | 3.41% |  |
Deferred interest |  |  |  |  | $ 511,000 | $ 760,000 | $ 411,000 |  |
Loans receivable in other current assets |  |  |  |  | 13,700,000 | 12,400,000 |  |  |
Loans receivable in other noncurrent assets |  |  |  |  | 35,300,000 | 38,800,000 |  |  |
Gain (loss) amortized in earning |  |  |  |  | 50,000 |  |  |  |
Available for sale equity securities |  |  |  |  |  |  | 32,900,000 |  |
Debt conversion ratio |  |  |  | 0.2492 |  |  |  |  |
Proceeds from sale of available for sale securities, net |  |  | $ 34,500,000 |  | 0 | 34,484,000 | 6,616,000 |  |
Gain on sale of available-for-sale securities, pre-tax |  |  | $ 1,600,000 |  |  |  |  |  |
Intangible assets, acquired wholesale natural gas energy |  |  |  |  | 10,000,000 |  |  |  |
Proceeds from sale-leaseback transaction |  |  |  |  | 4,000,000 | 9,895,000 | 7,820,000 |  |
Proceeds from sale leaseback transaction - solar |  |  |  |  | $ 42,927,000 | 0 | 71,538,000 |  |
Number of noncontributory defined benefit retirement plans (in plans) | plan |  |  |  |  | 2 |  |  |  |
Number of noncontributory medical and life insurance plans (in plans) | plan |  |  |  |  | 2 |  |  |  |
Operating lease assets |  |  |  |  | $ 131,769,000 | 0 |  |  |
Operating lease, liability |  |  |  |  | 101,754,000 |  |  |  |
Leaf River Energy Center LLC |  |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |
Operating lease assets |  |  |  |  |  |  |  | $ 21,600,000 |
Operating lease, liability |  |  |  |  |  |  |  | $ 21,600,000 |
Accounting Standards Update 2016-02 |  |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |
Operating lease assets |  |  |  |  |  |  |  |  | $ 67,100,000
Operating lease, liability |  |  |  |  |  |  |  |  | $ 67,100,000
Solar property | Nonutility |  |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |
Base gas cost |  |  |  |  | 674,930,000 | 593,867,000 |  |  |
Storage and transportation property | Nonutility |  |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |
Base gas cost |  |  |  |  | 428,491,000 | 28,445,000 |  |  |
Storage and transportation property, base gas | Nonutility |  |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |
Base gas cost |  |  |  |  | 5,700,000 |  |  |  |
Pension |  |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |
Employer discretionary contributions |  |  |  |  | 0 | 0 | 0 |  |
Employer contributions |  |  |  |  | 596,000 | 231,000 |  |  |
OPEB |  |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |
Employer contributions |  |  |  |  | 8,400,000 | 7,926,000 | 6,200,000 |  |
Clean Energy Ventures |  |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |
Proceeds from sale leaseback transaction - solar |  |  |  |  | 42,900,000 |  |  |  |
Talen Generation, LLC | Talen's Membership Interests In IEC |  |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |
Pipeline length owned | mi |  | 84 |  |  |  |  |  |  |
NJNG |  |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |
Proceeds from sale leaseback transaction - natural gas meters |  |  |  |  | $ 4,000,000 |  |  |  |
Proceeds from sale-leaseback transaction |  |  |  |  |  | $ 9,900,000 | $ 7,800,000 |  |
Common Units |  |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |
Other ownership interests, units issued (in shares) | shares |  |  |  | 1,840 |  |  |  |  |
Dominion Shares |  |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |
Other ownership interests, units issued (in shares) | shares |  |  |  | 458 |  |  |  |  |
Seven-Year Treasury Rate |  |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |
Debt instrument, term |  |  |  |  | 7 years |  |  |  |
Basis spread on variable rate |  |  |  |  | 0.60% |  |  |  |
Minimum |  |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |
Storage and pipeline capacity, contract term |  |  |  |  | 1 year |  |  |  |
Loans receivable term |  |  |  |  | 2 years |  |  |  |
Minimum | Solar property | Nonutility |  |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |
Estimated Useful Lives | 15 years |  |  |  | 15 years | 15 years | 15 years |  |
Minimum | Storage and transportation property | Nonutility |  |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |
Estimated Useful Lives |  |  |  |  | 5 years |  |  |  |
Minimum | Clean Energy Ventures |  |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |
Term of lease |  |  |  |  | 5 years |  |  |  |
Minimum | Talen Generation, LLC | Talen's Membership Interests In IEC |  |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |
Estimated Useful Lives |  |  |  |  | 5 years |  |  |  |
Minimum | NJNG |  |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |
Term of lease |  |  |  |  | 7 years |  |  |  |
Maximum |  |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |
Storage and pipeline capacity, contract term |  |  |  |  | 10 years |  |  |  |
Loans receivable term |  |  |  |  | 10 years |  |  |  |
Maximum | Solar property | Nonutility |  |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |
Estimated Useful Lives | 35 years |  |  |  | 35 years | 25 years | 25 years |  |
Maximum | Storage and transportation property | Nonutility |  |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |
Estimated Useful Lives |  |  |  |  | 50 years |  |  |  |
Maximum | Clean Energy Ventures |  |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |
Term of lease |  |  |  |  | 15 years |  |  |  |
Maximum | Talen Generation, LLC | Talen's Membership Interests In IEC |  |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |
Estimated Useful Lives |  |  |  |  | 30 years |  |  |  |
Maximum | NJNG |  |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |
Term of lease |  |  |  |  | 11 years |  |  |  |

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