Report of Business Segments (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021	Dec. 31, 2021
Segment Reporting Information [Line Items]
Operating revenues	$ 589.3	$ 1,630.6
Fuel, purchased power and direct transmission expense	255.7	1,346.8
Other operation and maintenance	115.0	109.3
Depreciation and amortization	107.4	98.7
Taxes other than income	28.1	27.2
OPERATING INCOME	83.1	48.6
Equity in earnings of unconsolidated affiliates	0.0	53.2
Other income (expense)	9.3	0.9
Interest expense	40.1	39.4
Income tax expense (benefit)	55.1	10.6
Net income (loss)	279.5	52.7
Investment in unconsolidated affiliates		409.2
Total assets	13,093.6	11,788.7	$ 12,606.4
Gain (Loss) on Disposition of Assets		53.2
Equity Securities, FV-NI, Gain (Loss)	282.3	0.0
Electric Utility [Member]
Segment Reporting Information [Line Items]
Operating revenues	589.3	1,630.6
Fuel, purchased power and direct transmission expense	255.7	1,346.8
Other operation and maintenance	115.5	110.3
Depreciation and amortization	107.4	98.7
Taxes other than income	26.8	25.7
OPERATING INCOME	83.9	49.1
Equity in earnings of unconsolidated affiliates		0.0
Other income (expense)	0.7	1.7
Interest expense	38.2	38.4
Income tax expense (benefit)	7.4	1.2
Net income (loss)	39.0	11.2
Investment in unconsolidated affiliates		0.0
Total assets	11,898.8	11,248.2
Equity Securities, FV-NI, Gain (Loss)	0.0
Natural Gas Midstream 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	0.7	0.4
Depreciation and amortization	0.0	0.0
Taxes other than income	0.0	0.1
OPERATING INCOME	(0.7)	(0.5)
Equity in earnings of unconsolidated affiliates		53.2
Other income (expense)	8.7	(0.5)
Interest expense	0.0	0.0
Income tax expense (benefit)	60.2	14.3
Net income (loss)	230.1	37.9
Investment in unconsolidated affiliates		409.2
Total assets	1,068.8	415.4
Equity Securities, FV-NI, Gain (Loss)	282.3
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	(1.2)	(1.4)
Depreciation and amortization	0.0	0.0
Taxes other than income	1.3	1.4
OPERATING INCOME	(0.1)	0.0
Equity in earnings of unconsolidated affiliates		0.0
Other income (expense)	0.0	(0.1)
Interest expense	2.0	1.2
Income tax expense (benefit)	(12.5)	(4.9)
Net income (loss)	10.4	3.6
Investment in unconsolidated affiliates		0.0
Total assets	320.9	521.6
Equity Securities, FV-NI, Gain (Loss)	0.0
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
Equity in earnings of unconsolidated affiliates		0.0
Other income (expense)	(0.1)	(0.2)
Interest expense	(0.1)	(0.2)
Income tax expense (benefit)	0.0	0.0
Net income (loss)	0.0	0.0
Investment in unconsolidated affiliates		0.0
Total assets	(194.9)	$ (396.5)
Equity Securities, FV-NI, Gain (Loss)	$ 0.0