FAIR VALUE MEASUREMENTS - Fixed Income Securities Held in Nuclear Trusts (Details) - Spent Nuclear Fuel and Decommissioning Trusts $ in Millions	Sep. 30, 2025 USD ($)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Within 1 year	$ 366.1
After 1 year through 5 years	736.7
After 5 years through 10 years	209.0
After 10 years	426.2
Total	1,738.0
I&M
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Within 1 year	366.1
After 1 year through 5 years	736.7
After 5 years through 10 years	209.0
After 10 years	426.2
Total	$ 1,738.0

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