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FAIR VALUE MEASUREMENTS - Schedule of Long-term Debt Redemption and Issuance (Details) - USD ($) $ in Millions	1 Months Ended	9 Months Ended
	Apr. 30, 2026	Sep. 30, 2026	Sep. 30, 2025
Debt Instrument [Line Items]
Redemptions		$ 75	$ 625
Issuances		$ 425	$ 1,475
Senior Unsecured Notes Maturing 2026 At 4.00% \| Unsecured Debt \| ATSI
Debt Instrument [Line Items]
Interest Rate	4.00%
Redemptions	$ 75
Senior Unsecured Notes Maturing 2036 At 5.02% \| Unsecured Debt \| MAIT
Debt Instrument [Line Items]
Interest Rate	5.02%
Issuances	$ 250
Senior Unsecured Notes Maturing 2033 At 5.19% \| Unsecured Debt \| ATSI
Debt Instrument [Line Items]
Interest Rate	5.19%
Issuances	$ 175