COMPREHENSIVE INCOME - Schedule of Comprehensive Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 25,285.9	$ 24,122.4	$ 22,680.2
TOTAL OTHER COMPREHENSIVE INCOME (LOSS)	52.4	(139.2)	(101.1)
Ending balance	26,986.1	25,285.9	24,122.4
Total
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(55.5)	83.7	184.8
Change in Fair Value Recognized in AOCI, Net of Tax	50.6	(198.5)	340.3
Amount of (Gain) Loss Reclassified from AOCI	2.2	53.8	(537.7)
Income Tax (Expense) Benefit	0.4	11.2	(113.0)
Reclassifications from AOCI, Net of Income Tax (Expense) Benefit	1.8	42.6	(424.7)
Reclassifications of KPCo Pension and OPEB Regulatory Assets to AOCI, before Income Tax (Expense) Benefit		21.1	(21.1)
Income Tax (Expense) Benefit		4.4	(4.4)
Reclassifications of KPCo Pension and OPEB Regulatory Assets to AOCI, Net of Income Tax (Expense) Benefit		16.7	(16.7)
TOTAL OTHER COMPREHENSIVE INCOME (LOSS)	52.4	(139.2)	(101.1)
Ending balance	(3.1)	(55.5)	83.7
Cash Flow Hedges | Revenues
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amount of (Gain) Loss Reclassified from AOCI			0.1
Cash Flow Hedges | Purchased Electricity, Fuel and Other Consumables Used for Electric Generation
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amount of (Gain) Loss Reclassified from AOCI			(528.6)
Pension and OPEB Amortization of Deferred Costs
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	92.6	105.2	115.6
Change in Fair Value Recognized in AOCI, Net of Tax	0.0	0.0	0.0
Amount of (Gain) Loss Reclassified from AOCI	8.1	(16.0)	(13.2)
Income Tax (Expense) Benefit	1.7	(3.4)	(2.8)
Reclassifications from AOCI, Net of Income Tax (Expense) Benefit	6.4	(12.6)	(10.4)
TOTAL OTHER COMPREHENSIVE INCOME (LOSS)	6.4	(12.6)	(10.4)
Ending balance	99.0	92.6	105.2
Amortization of Prior Service Cost (Credit)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amount of (Gain) Loss Reclassified from AOCI	(5.4)	(21.2)	(21.8)
Amortization of Actuarial (Gains) Losses
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amount of (Gain) Loss Reclassified from AOCI	2.2	5.2	8.6
Recognition of Pension Settlement Costs
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amount of (Gain) Loss Reclassified from AOCI	11.3
Pension and OPEB Changes in Funded Status
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(244.9)	(245.3)	(73.2)
Change in Fair Value Recognized in AOCI, Net of Tax	41.0	(16.3)	(155.4)
Reclassifications of KPCo Pension and OPEB Regulatory Assets to AOCI, before Income Tax (Expense) Benefit		21.1	(21.1)
Income Tax (Expense) Benefit		4.4	(4.4)
Reclassifications of KPCo Pension and OPEB Regulatory Assets to AOCI, Net of Income Tax (Expense) Benefit		16.7	(16.7)
TOTAL OTHER COMPREHENSIVE INCOME (LOSS)	41.0	0.4	(172.1)
Ending balance	(203.9)	(244.9)	(245.3)
Commodity | Cash Flow Hedges
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	104.9	223.5	163.7
Change in Fair Value Recognized in AOCI, Net of Tax	2.5	(175.8)	477.3
Amount of (Gain) Loss Reclassified from AOCI	(11.2)	72.2	(528.5)
Income Tax (Expense) Benefit	(2.3)	15.0	(111.0)
Reclassifications from AOCI, Net of Income Tax (Expense) Benefit	(8.9)	57.2	(417.5)
TOTAL OTHER COMPREHENSIVE INCOME (LOSS)	(6.4)	(118.6)	59.8
Ending balance	98.5	104.9	223.5
Commodity | Cash Flow Hedges | Revenues
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amount of (Gain) Loss Reclassified from AOCI			0.1
Commodity | Cash Flow Hedges | Purchased Electricity, Fuel and Other Consumables Used for Electric Generation
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amount of (Gain) Loss Reclassified from AOCI			(528.6)
Interest Rate on Long-term Debt (USD) | Cash Flow Hedges
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(8.1)	0.3	(21.3)
Change in Fair Value Recognized in AOCI, Net of Tax	7.1	(6.4)	18.4
Amount of (Gain) Loss Reclassified from AOCI	5.3	(2.4)	4.0
Income Tax (Expense) Benefit	1.0	(0.4)	0.8
Reclassifications from AOCI, Net of Income Tax (Expense) Benefit	4.3	(2.0)	3.2
TOTAL OTHER COMPREHENSIVE INCOME (LOSS)	11.4	(8.4)	21.6
Ending balance	$ 3.3	$ (8.1)	$ 0.3