Comprehensive Income (Details) - USD ($) $ in Millions		3 Months Ended						9 Months Ended
		Sep. 30, 2024	Jun. 30, 2024	Mar. 31, 2024	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023	Sep. 30, 2024	Sep. 30, 2023
Changes in Accumulated Other Comprehensive Income (Loss) by Component
Beginning Balance in AOCI		$ (40.0)		$ (55.5)	$ (36.4)		$ 83.7	$ (55.5)	$ 83.7
Change in Fair Value Recognized in AOCI		(53.3)			12.4			(21.5)	(177.7)
Commodity
Generation & Marketing Revenues		5,420.1			5,341.7			15,025.0	14,405.1
Interest Rate
Interest Expense, Operating and Nonoperating		498.8			470.3			1,400.0	1,346.0
Amortization Of Pension And Other Postretirement Benefit Plans
Reclassification from AOCI, before Income Tax (Expense) Benefit		8.2			(20.9)			(12.5)	46.6
Income Tax (Expense) Benefit		37.5			64.2			(97.7)	103.2
Reclassification from AOCI, Net of Income Tax (Expense) Benefit		6.4			(16.5)			(9.9)	36.8
Reclassifications of KPCo Pension and OPEB Regulatory Assets, Net of Tax		0.0			0.0			0.0	16.7
Net Current Period Other Comprehensive Income (Loss)		(46.9)	$ 22.3	(6.8)	(4.1)	$ 31.7	(151.8)	(31.4)	(124.2)
Ending Balance in AOCI		(86.9)	(40.0)		(40.5)	(36.4)		(86.9)	(40.5)
Reclassification of KPCo Pension and OPEB Regulatory Assets from AOCI
Amortization Of Pension And Other Postretirement Benefit Plans
Income Tax (Expense) Benefit									4.4
Reclassifications of KPCo Pension and OPEB Regulatory Assets from AOCI, before Income Tax (Expense) Benefit									21.1
Reclassifications of KPCo Pension and OPEB Regulatory Assets, Net of Tax									16.7
Cash Flow Hedges [Member] | Commodity [Member]
Changes in Accumulated Other Comprehensive Income (Loss) by Component
Beginning Balance in AOCI		103.7		104.9	93.5		223.5	104.9	223.5
Change in Fair Value Recognized in AOCI		(28.8)			19.3			(16.3)	(170.1)
Amortization Of Pension And Other Postretirement Benefit Plans
Reclassification from AOCI, before Income Tax (Expense) Benefit		0.1			(15.6)			(17.4)	59.7
Reclassification from AOCI, Net of Income Tax (Expense) Benefit		0.0			(12.2)			(13.7)	47.2
Net Current Period Other Comprehensive Income (Loss)		(28.8)			7.1			(30.0)	(122.9)
Ending Balance in AOCI		74.9	103.7		100.6	93.5		74.9	100.6
Cash Flow Hedges [Member] | Commodity [Member] | Reclassification of KPCo Pension and OPEB Regulatory Assets from AOCI
Amortization Of Pension And Other Postretirement Benefit Plans
Income Tax (Expense) Benefit									0.0
Reclassifications of KPCo Pension and OPEB Regulatory Assets from AOCI, before Income Tax (Expense) Benefit									0.0
Reclassifications of KPCo Pension and OPEB Regulatory Assets, Net of Tax									0.0
Cash Flow Hedges [Member] | Interest Rate [Member]
Changes in Accumulated Other Comprehensive Income (Loss) by Component
Beginning Balance in AOCI		9.3		(8.1)	12.7		0.3	(8.1)	0.3
Change in Fair Value Recognized in AOCI		(24.5)			(6.9)			(5.2)	5.3
Amortization Of Pension And Other Postretirement Benefit Plans
Reclassification from AOCI, before Income Tax (Expense) Benefit		8.8			(1.3)			6.5	(1.1)
Reclassification from AOCI, Net of Income Tax (Expense) Benefit		7.0			(1.1)			5.1	(0.9)
Net Current Period Other Comprehensive Income (Loss)		(17.5)			(8.0)			(0.1)	4.4
Ending Balance in AOCI		(8.2)	9.3		4.7	12.7		(8.2)	4.7
Cash Flow Hedges [Member] | Interest Rate [Member] | Reclassification of KPCo Pension and OPEB Regulatory Assets from AOCI
Amortization Of Pension And Other Postretirement Benefit Plans
Income Tax (Expense) Benefit									0.0
Reclassifications of KPCo Pension and OPEB Regulatory Assets from AOCI, before Income Tax (Expense) Benefit									0.0
Reclassifications of KPCo Pension and OPEB Regulatory Assets, Net of Tax									0.0
Pension and OPEB [Member]
Changes in Accumulated Other Comprehensive Income (Loss) by Component
Beginning Balance in AOCI		(153.0)		$ (152.3)	(142.6)		$ (140.1)	(152.3)	(140.1)
Change in Fair Value Recognized in AOCI		0.0			0.0			0.0	(12.9)
Amortization Of Pension And Other Postretirement Benefit Plans
Reclassification from AOCI, before Income Tax (Expense) Benefit		(0.7)			(4.0)			(1.6)	(12.0)
Reclassification from AOCI, Net of Income Tax (Expense) Benefit		(0.6)			(3.2)			(1.3)	(9.5)
Net Current Period Other Comprehensive Income (Loss)		(0.6)			(3.2)			(1.3)	(5.7)
Ending Balance in AOCI		(153.6)	$ (153.0)		(145.8)	$ (142.6)		(153.6)	(145.8)
Pension and OPEB [Member] | Reclassification of KPCo Pension and OPEB Regulatory Assets from AOCI | Pension Costs
Amortization Of Pension And Other Postretirement Benefit Plans
Income Tax (Expense) Benefit									4.4
Reclassifications of KPCo Pension and OPEB Regulatory Assets from AOCI, before Income Tax (Expense) Benefit									21.1
Reclassifications of KPCo Pension and OPEB Regulatory Assets, Net of Tax									16.7
Generation and Marketing [Member]
Commodity
Generation & Marketing Revenues		483.7			527.5			1,442.1	1,172.6
Reclassifications from Accumulated Other Comprehensive Income [Member]
Interest Rate
Interest Expense, Operating and Nonoperating	[1]	8.8			(1.3)			6.5	(1.1)
Amortization Of Pension And Other Postretirement Benefit Plans
Amortization of Prior Service Cost (Credit)		(1.4)			(5.3)			(4.0)	(15.9)
Amortization of Actuarial (Gains) Losses		0.7			1.3			2.4	3.9
Income Tax (Expense) Benefit		1.8			(4.4)			(2.6)	9.8
Reclassifications from Accumulated Other Comprehensive Income [Member] | Cash Flow Hedges [Member] | Commodity [Member]
Interest Rate
Interest Expense, Operating and Nonoperating	[1]	0.0			0.0			0.0	0.0
Amortization Of Pension And Other Postretirement Benefit Plans
Amortization of Prior Service Cost (Credit)		0.0			0.0			0.0	0.0
Amortization of Actuarial (Gains) Losses		0.0			0.0			0.0	0.0
Income Tax (Expense) Benefit		0.1			(3.4)			(3.7)	12.5
Reclassifications from Accumulated Other Comprehensive Income [Member] | Cash Flow Hedges [Member] | Interest Rate [Member]
Interest Rate
Interest Expense, Operating and Nonoperating	[1]	8.8			(1.3)			6.5	(1.1)
Amortization Of Pension And Other Postretirement Benefit Plans
Amortization of Prior Service Cost (Credit)		0.0			0.0			0.0	0.0
Amortization of Actuarial (Gains) Losses		0.0			0.0			0.0	0.0
Income Tax (Expense) Benefit		1.8			(0.2)			1.4	(0.2)
Reclassifications from Accumulated Other Comprehensive Income [Member] | Pension and OPEB [Member]
Interest Rate
Interest Expense, Operating and Nonoperating	[1]	0.0			0.0			0.0	0.0
Amortization Of Pension And Other Postretirement Benefit Plans
Amortization of Prior Service Cost (Credit)		(1.4)			(5.3)			(4.0)	(15.9)
Amortization of Actuarial (Gains) Losses		0.7			1.3			2.4	3.9
Income Tax (Expense) Benefit		(0.1)			(0.8)			(0.3)	(2.5)
Reclassifications from Accumulated Other Comprehensive Income [Member] | Purchased Electricity, Fuel and Other Consumables Used for Electric Generation [Member]
Commodity
Purchased Electricity, Fuel and Other Consumables Used for Electric Generation	[1]	0.1			(15.6)			(17.4)	59.7
Reclassifications from Accumulated Other Comprehensive Income [Member] | Purchased Electricity, Fuel and Other Consumables Used for Electric Generation [Member] | Cash Flow Hedges [Member] | Commodity [Member]
Commodity
Purchased Electricity, Fuel and Other Consumables Used for Electric Generation	[1]	0.1			(15.6)			(17.4)	59.7
Reclassifications from Accumulated Other Comprehensive Income [Member] | Purchased Electricity, Fuel and Other Consumables Used for Electric Generation [Member] | Cash Flow Hedges [Member] | Interest Rate [Member]
Commodity
Purchased Electricity, Fuel and Other Consumables Used for Electric Generation	[1]	0.0			0.0			0.0	0.0
Reclassifications from Accumulated Other Comprehensive Income [Member] | Purchased Electricity, Fuel and Other Consumables Used for Electric Generation [Member] | Pension and OPEB [Member]
Commodity
Purchased Electricity, Fuel and Other Consumables Used for Electric Generation	[1]	$ 0.0			$ 0.0			$ 0.0	$ 0.0

[1] Amounts reclassified to the referenced line item on the statements of income.