Report of Business Segments (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2022	Mar. 31, 2022	Jun. 30, 2021	Mar. 31, 2021	Jun. 30, 2022	Jun. 30, 2021	Dec. 31, 2021
Segment Reporting Information [Line Items]
Operating revenues	$ 803.7		$ 577.4		$ 1,393.0	$ 2,208.0
Fuel, purchased power and direct transmission expense	393.3		200.0		649.0	1,546.8
Other operation and maintenance	118.1		119.2		233.1	228.5
Depreciation and amortization	111.6		102.9		219.0	201.6
Taxes other than income	24.1		25.8		52.2	53.0
OPERATING INCOME	156.6		129.5		239.7	178.1
Income (Loss) from Equity Method Investments	0.0		33.5		0.0	86.7
Other income (expense)	11.7		(0.1)		21.0	0.8
Interest expense	42.4		39.9		82.5	79.3
Income tax expense (benefit)	13.2		10.1		68.3	20.7
Net income (loss)	73.1	$ 279.5	112.9	$ 52.7	352.6	165.6
Investment in unconsolidated affiliates			425.4			425.4
Total assets	12,838.0		11,899.5		12,838.0	11,899.5	$ 12,606.4
Gain (Loss) on Disposition of Assets						86.7
Equity Securities, FV-NI, Gain (Loss)	(39.6)		0.0		242.7	0.0
Electric Utility [Member]
Segment Reporting Information [Line Items]
Operating revenues	803.7		577.4		1,393.0	2,208.0
Fuel, purchased power and direct transmission expense	393.3		200.0		649.0	1,546.8
Other operation and maintenance	118.3		118.3		233.8	228.6
Depreciation and amortization	111.6		102.9		219.0	201.6
Taxes other than income	23.3		25.0		50.1	50.7
OPERATING INCOME	157.2		131.2		241.1	180.3
Income (Loss) from Equity Method Investments			0.0			0.0
Other income (expense)	(0.3)		1.2		0.4	2.9
Interest expense	39.8		37.9		78.0	76.3
Income tax expense (benefit)	16.4		9.4		23.8	10.6
Net income (loss)	100.7		85.1		139.7	96.3
Investment in unconsolidated affiliates			0.0			0.0
Total assets	12,317.2		11,459.3		12,317.2	11,459.3
Equity Securities, FV-NI, Gain (Loss)	0.0				0.0
Natural Gas Midstream Operations [Member]
Segment Reporting Information [Line Items]
Operating revenues	0.0		0.0		0.0	0.0
Fuel, purchased power and direct transmission expense	0.0		0.0		0.0	0.0
Other operation and maintenance	0.5		1.6		1.2	2.0
Depreciation and amortization	0.0		0.0		0.0	0.0
Taxes other than income	0.1		0.1		0.1	0.2
OPERATING INCOME	(0.6)		(1.7)		(1.3)	(2.2)
Income (Loss) from Equity Method Investments			33.5			86.7
Other income (expense)	12.4		(0.2)		21.1	(0.7)
Interest expense	0.0		0.0		0.0	0.0
Income tax expense (benefit)	(8.9)		(0.6)		51.3	13.7
Net income (loss)	(18.9)		32.2		211.2	70.1
Investment in unconsolidated affiliates			425.4			425.4
Total assets	401.7		427.8		401.7	427.8
Equity Securities, FV-NI, Gain (Loss)	(39.6)				242.7
Other Operations [Member]
Segment Reporting Information [Line Items]
Operating revenues	0.0		0.0		0.0	0.0
Fuel, purchased power and direct transmission expense	0.0		0.0		0.0	0.0
Other operation and maintenance	(0.7)		(0.7)		(1.9)	(2.1)
Depreciation and amortization	0.0		0.0		0.0	0.0
Taxes other than income	0.7		0.7		2.0	2.1
OPERATING INCOME	0.0		0.0		(0.1)	0.0
Income (Loss) from Equity Method Investments			0.0			0.0
Other income (expense)	0.0		(0.5)		0.0	(0.6)
Interest expense	3.0		2.6		5.0	3.8
Income tax expense (benefit)	5.7		1.3		(6.8)	(3.6)
Net income (loss)	(8.7)		(4.4)		1.7	(0.8)
Investment in unconsolidated affiliates			0.0			0.0
Total assets	751.9		127.4		751.9	127.4
Equity Securities, FV-NI, Gain (Loss)	0.0				0.0
Eliminations [Member]
Segment Reporting Information [Line Items]
Operating revenues	0.0		0.0		0.0	0.0
Fuel, purchased power and direct transmission expense	0.0		0.0		0.0	0.0
Other operation and maintenance	0.0		0.0		0.0	0.0
Depreciation and amortization	0.0		0.0		0.0	0.0
Taxes other than income	0.0		0.0		0.0	0.0
OPERATING INCOME	0.0		0.0		0.0	0.0
Income (Loss) from Equity Method Investments			0.0			0.0
Other income (expense)	(0.4)		(0.6)		(0.5)	(0.8)
Interest expense	(0.4)		(0.6)		(0.5)	(0.8)
Income tax expense (benefit)	0.0		0.0		0.0	0.0
Net income (loss)	0.0		0.0		0.0	0.0
Investment in unconsolidated affiliates			0.0			0.0
Total assets	(632.8)		$ (115.0)		(632.8)	$ (115.0)
Equity Securities, FV-NI, Gain (Loss)	$ 0.0				$ 0.0