COMMITMENTS, GUARANTEES AND CONTINGENCIES - Nuclear Insurance, Commitments and Collateral (Details)	3 Months Ended
Dec. 31, 2020 USD ($)
Loss Contingencies [Line Items]
Outstanding guarantees and other assurances aggregated	$ 1,700,000,000
Company posted collateral related to net liability positions	20,000,000
Collateral posted due to credit rating downgrade	19,000,000
Subsidiaries' Guarantees
Loss Contingencies [Line Items]
Outstanding guarantees and other assurances aggregated	1,100,000,000
Other Guarantees
Loss Contingencies [Line Items]
Outstanding guarantees and other assurances aggregated	108,000,000
Other Assurances
Loss Contingencies [Line Items]
Outstanding guarantees and other assurances aggregated	490,000,000
Global Holding \| Senior Secured Term Loan \| Senior Loans
Loss Contingencies [Line Items]
Long-term debt and other long-term obligations	$ 108,000,000
Global Holding \| Signal Peak, Global Rail and Affiliates \| Senior Secured Term Loan \| Senior Loans
Loss Contingencies [Line Items]
Investment ownership percentage	69.99%
FEV \| Signal Peak \| Senior Secured Term Loan \| Senior Loans \| Global Holding
Loss Contingencies [Line Items]
Investment ownership percentage	33.33%
WMB Marketing Ventures, LLC \| Signal Peak \| Senior Secured Term Loan \| Senior Loans \| Global Holding
Loss Contingencies [Line Items]
Investment ownership percentage	33.33%
Term Loan Facility Due March 2020 \| Line of Credit \| Global Holding
Loss Contingencies [Line Items]
Face amount of loan	$ 120,000,000

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