FAIR VALUE MEASUREMENTS - Summary of Nuclear Trust Investments (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2025	Dec. 31, 2024
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	$ 4,309.3	$ 4,395.1
Spent Nuclear Fuel and Decommissioning Trusts
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gross Unrealized Gains	2,183.6	2,297.8
Gross Unrealized Losses	(12.9)	(15.5)
Other-Than-Temporary Impairments	(25.1)	(26.0)
I&M
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	4,309.3	4,395.1
I&M | Spent Nuclear Fuel and Decommissioning Trusts
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gross Unrealized Gains	2,183.6	2,297.8
Gross Unrealized Losses	(12.9)	(15.5)
Other-Than-Temporary Impairments	(25.1)	(26.0)
Cash and Cash Equivalents
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	24.6	23.3
Cash and Cash Equivalents | Spent Nuclear Fuel and Decommissioning Trusts
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Other-Than-Temporary Impairments	0.0	0.0
Cash and Cash Equivalents | I&M
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	24.6	23.3
Cash and Cash Equivalents | I&M | Spent Nuclear Fuel and Decommissioning Trusts
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Other-Than-Temporary Impairments	0.0	0.0
United States Government
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	1,327.6	1,322.8
United States Government | Spent Nuclear Fuel and Decommissioning Trusts
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gross Unrealized Gains	19.2	8.2
Gross Unrealized Losses	(1.2)	(5.3)
Other-Than-Temporary Impairments	(19.0)	(20.2)
United States Government | I&M
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	1,327.6	1,322.8
United States Government | I&M | Spent Nuclear Fuel and Decommissioning Trusts
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gross Unrealized Gains	19.2	8.2
Gross Unrealized Losses	(1.2)	(5.3)
Other-Than-Temporary Impairments	(19.0)	(20.2)
Corporate Debt
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	240.5	211.3
Corporate Debt | Spent Nuclear Fuel and Decommissioning Trusts
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gross Unrealized Gains	1.6	0.7
Gross Unrealized Losses	(8.6)	(9.8)
Other-Than-Temporary Impairments	(6.1)	(5.8)
Corporate Debt | I&M
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	240.5	211.3
Corporate Debt | I&M | Spent Nuclear Fuel and Decommissioning Trusts
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gross Unrealized Gains	1.6	0.7
Gross Unrealized Losses	(8.6)	(9.8)
Other-Than-Temporary Impairments	(6.1)	(5.8)
State and Local Government
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	0.0	0.0
State and Local Government | Spent Nuclear Fuel and Decommissioning Trusts
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Other-Than-Temporary Impairments	0.0	0.0
State and Local Government | I&M
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	0.0	0.0
State and Local Government | I&M | Spent Nuclear Fuel and Decommissioning Trusts
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Other-Than-Temporary Impairments	0.0	0.0
Subtotal Fixed Income Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	1,568.1	1,534.1
Subtotal Fixed Income Securities | Spent Nuclear Fuel and Decommissioning Trusts
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gross Unrealized Gains	20.8	8.9
Gross Unrealized Losses	(9.8)	(15.1)
Other-Than-Temporary Impairments	(25.1)	(26.0)
Subtotal Fixed Income Securities | I&M
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	1,568.1	1,534.1
Subtotal Fixed Income Securities | I&M | Spent Nuclear Fuel and Decommissioning Trusts
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gross Unrealized Gains	20.8	8.9
Gross Unrealized Losses	(9.8)	(15.1)
Other-Than-Temporary Impairments	(25.1)	(26.0)
Equity Securities - Domestic
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	2,716.6	2,837.7
Equity Securities - Domestic | Spent Nuclear Fuel and Decommissioning Trusts
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gross Unrealized Gains	2,162.8	2,288.9
Gross Unrealized Losses	(3.1)	(0.4)
Other-Than-Temporary Impairments	0.0	0.0
Equity Securities - Domestic | I&M
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	2,716.6	2,837.7
Equity Securities - Domestic | I&M | Spent Nuclear Fuel and Decommissioning Trusts
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gross Unrealized Gains	2,162.8	2,288.9
Gross Unrealized Losses	(3.1)	(0.4)
Other-Than-Temporary Impairments	$ 0.0	$ 0.0