Report of Business Segments (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022	Dec. 31, 2022
Segment Reporting Information [Line Items]
Operating revenues	$ 557.2	$ 589.3
Fuel, purchased power and direct transmission expense	200.6	255.7
Other operation and maintenance	130.4	115.0
Depreciation and amortization	121.2	107.4
Taxes other than income	28.5	28.1
OPERATING INCOME	76.5	83.1
Gain on equity securities	0.0	282.3
Other income (expense)	16.4	9.3
Interest expense	47.8	40.1
Income tax expense	6.8	55.1
Net income (loss)	38.3	279.5
Total assets	12,790.3	13,093.6	$ 12,544.7
Eliminations [Member]
Segment Reporting Information [Line Items]
Operating revenues	0.0	0.0
Fuel, purchased power and direct transmission expense	0.0	0.0
Other operation and maintenance	0.0	0.0
Depreciation and amortization	0.0	0.0
Taxes other than income	0.0	0.0
OPERATING INCOME	0.0	0.0
Gain on equity securities		0.0
Other income (expense)	(3.4)	(0.1)
Interest expense	(3.4)	(0.1)
Income tax expense	0.0	0.0
Net income (loss)	0.0	0.0
Total assets	(559.5)	(194.9)
Electrics Company [Member]
Segment Reporting Information [Line Items]
Operating revenues	557.2	589.3
Fuel, purchased power and direct transmission expense	200.6	255.7
Other operation and maintenance	131.5	115.5
Depreciation and amortization	121.2	107.4
Taxes other than income	27.1	26.8
OPERATING INCOME	76.8	83.9
Gain on equity securities		0.0
Other income (expense)	15.3	0.7
Interest expense	45.7	38.2
Income tax expense	6.6	7.4
Net income (loss)	39.8	39.0
Total assets	12,762.7	11,898.8
Natural Gas Midstream Operations [Member]
Segment Reporting Information [Line Items]
Operating revenues		0.0
Fuel, purchased power and direct transmission expense		0.0
Other operation and maintenance		0.7
Depreciation and amortization		0.0
Taxes other than income		0.0
OPERATING INCOME		(0.7)
Gain on equity securities		282.3
Other income (expense)		8.7
Interest expense		0.0
Income tax expense		60.2
Net income (loss)		230.1
Total assets		1,068.8
Other Operations [Member]
Segment Reporting Information [Line Items]
Operating revenues	0.0	0.0
Fuel, purchased power and direct transmission expense	0.0	0.0
Other operation and maintenance	(1.1)	(1.2)
Depreciation and amortization	0.0	0.0
Taxes other than income	1.4	1.3
OPERATING INCOME	(0.3)	(0.1)
Gain on equity securities		0.0
Other income (expense)	4.5	0.0
Interest expense	5.5	2.0
Income tax expense	0.2	(12.5)
Net income (loss)	(1.5)	10.4
Total assets	$ 587.1	$ 320.9