COMPREHENSIVE INCOME (Details) - USD ($) $ in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2025	Jun. 30, 2025	Mar. 31, 2025	Sep. 30, 2024	Jun. 30, 2024	Mar. 31, 2024	Sep. 30, 2025	Sep. 30, 2024
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	$ 30,968.4	$ 27,364.0	$ 26,986.1	$ 26,175.7	$ 25,843.7	$ 25,285.9	$ 26,986.1	$ 25,285.9
TOTAL OTHER COMPREHENSIVE INCOME (LOSS)	(15.2)	(32.9)	23.4	(46.9)	22.3	(6.8)	(24.7)	(31.4)
Ending balance	31,479.3	30,968.4	27,364.0	26,658.8	26,175.7	25,843.7	31,479.3	26,658.8
Total
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	(12.6)	20.3	(3.1)	(40.0)	(62.3)	(55.5)	(3.1)	(55.5)
Change in Fair Value Recognized in AOCI, Net of Tax	(17.2)			(53.3)			(16.0)	(21.5)
Amount of (Gain) Loss Reclassified from AOCI	2.6			8.2			(11.0)	(12.5)
Income Tax (Expense) Benefit	0.6			1.8			(2.3)	(2.6)
Reclassifications from AOCI, Net of Income Tax (Expense) Benefit	2.0			6.4			(8.7)	(9.9)
TOTAL OTHER COMPREHENSIVE INCOME (LOSS)	(15.2)	(32.9)	23.4	(46.9)	22.3	(6.8)	(24.7)	(31.4)
Ending balance	(27.8)	(12.6)	20.3	(86.9)	(40.0)	(62.3)	(27.8)	(86.9)
Pension and OPEB
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	(104.3)		(104.9)	(153.0)		(152.3)	(104.9)	(152.3)
Change in Fair Value Recognized in AOCI, Net of Tax	0.0			0.0			0.0	0.0
Amount of (Gain) Loss Reclassified from AOCI	0.3			(0.7)			1.0	(1.6)
Income Tax (Expense) Benefit	0.1			(0.1)			0.2	(0.3)
Reclassifications from AOCI, Net of Income Tax (Expense) Benefit	0.2			(0.6)			0.8	(1.3)
TOTAL OTHER COMPREHENSIVE INCOME (LOSS)	0.2			(0.6)			0.8	(1.3)
Ending balance	(104.1)	(104.3)		(153.6)	(153.0)		(104.1)	(153.6)
Amortization of Prior Service Cost (Credit)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Amount of (Gain) Loss Reclassified from AOCI	(0.2)			(1.4)			(0.6)	(4.0)
Amortization of Actuarial (Gains) Losses
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Amount of (Gain) Loss Reclassified from AOCI	0.5			0.7			1.6	2.4
Commodity | Cash Flow Hedges
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	91.5		98.5	103.7		104.9	98.5	104.9
Change in Fair Value Recognized in AOCI, Net of Tax	(17.1)			(28.8)			(14.9)	(16.3)
Amount of (Gain) Loss Reclassified from AOCI	3.2			0.1			(8.5)	(17.4)
Income Tax (Expense) Benefit	0.7			0.1			(1.8)	(3.7)
Reclassifications from AOCI, Net of Income Tax (Expense) Benefit	2.5			0.0			(6.7)	(13.7)
TOTAL OTHER COMPREHENSIVE INCOME (LOSS)	(14.6)			(28.8)			(21.6)	(30.0)
Ending balance	76.9	91.5		74.9	103.7		76.9	74.9
Commodity | Cash Flow Hedges | Purchased Electricity, Fuel and Other Consumables Used for Electric Generation
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Amount of (Gain) Loss Reclassified from AOCI	3.2			0.1			(8.5)	(17.4)
Interest Rate | Cash Flow Hedges
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	0.2		$ 3.3	9.3		$ (8.1)	3.3	(8.1)
Change in Fair Value Recognized in AOCI, Net of Tax	(0.1)			(24.5)			(1.1)	(5.2)
Amount of (Gain) Loss Reclassified from AOCI	(0.9)			8.8			(3.5)	6.5
Income Tax (Expense) Benefit	(0.2)			1.8			(0.7)	1.4
Reclassifications from AOCI, Net of Income Tax (Expense) Benefit	(0.7)			7.0			(2.8)	5.1
TOTAL OTHER COMPREHENSIVE INCOME (LOSS)	(0.8)			(17.5)			(3.9)	(0.1)
Ending balance	(0.6)	$ 0.2		(8.2)	$ 9.3		(0.6)	(8.2)
Interest Rate | Cash Flow Hedges | Interest Expense
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Amount of (Gain) Loss Reclassified from AOCI	$ (0.9)			$ 8.8			$ (3.5)	$ 6.5