Report of Business Segments (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2021	Mar. 31, 2021	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2021	Jun. 30, 2020		Dec. 31, 2020
Segment Reporting Information [Line Items]
Operating revenues	$ 577.4		$ 503.5		$ 2,208.0	$ 934.8
Fuel, purchased power and direct transmission expense	200.0		137.4		1,546.8	272.4
Other operation and maintenance	119.2		117.5		228.5	237.5
Depreciation and amortization	102.9		97.3		201.6	191.7
Taxes other than income	25.8		25.9		53.0	51.5
OPERATING INCOME	129.5		125.4		178.1	181.7
Equity in earnings of unconsolidated affiliates	33.5		26.9		86.7	(719.6)	[1]
Other Nonoperating Income (Expense)	(0.1)		2.0		0.8	4.1
Interest expense	39.9		40.8		79.3	79.1
Income Tax Expense (Benefit)	10.1		27.6		20.7	(207.0)
Net income (loss)	112.9	$ 52.7	85.9	$ (491.8)	165.6	(405.9)
Investment in unconsolidated affiliates	451.7		376.5		451.7	376.5		$ 397.4
Total assets	11,899.5		10,439.1		11,899.5	10,439.1		$ 10,718.8
Electric Utility [Member]
Segment Reporting Information [Line Items]
Operating revenues	577.4		503.5		2,208.0	934.8
Fuel, purchased power and direct transmission expense	200.0		137.4		1,546.8	272.4
Other operation and maintenance	118.3		117.7		228.6	238.7
Depreciation and amortization	102.9		97.3		201.6	191.7
Taxes other than income	25.0		25.0		50.7	48.9
OPERATING INCOME	131.2		126.1		180.3	183.1
Equity in earnings of unconsolidated affiliates	0.0		0.0		0.0	0.0	[1]
Other Nonoperating Income (Expense)	1.2		0.5		2.9	2.3
Interest expense	37.9		39.3		76.3	76.2
Income Tax Expense (Benefit)	9.4		8.4		10.6	10.4
Net income (loss)	85.1		78.9		96.3	98.8
Investment in unconsolidated affiliates	0.0		0.0		0.0	0.0
Total assets	11,459.3		10,367.6		11,459.3	10,367.6
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	1.6		0.3		2.0	0.9
Depreciation and amortization	0.0		0.0		0.0	0.0
Taxes other than income	0.1		0.1		0.2	0.2
OPERATING INCOME	(1.7)		(0.4)		(2.2)	(1.1)
Equity in earnings of unconsolidated affiliates	33.5		26.9		86.7	(719.6)	[1]
Other Nonoperating Income (Expense)	(0.2)		0.0		(0.7)	0.0
Interest expense	0.0		0.0		0.0	0.0
Income Tax Expense (Benefit)	(0.6)		7.5		13.7	(171.7)
Net income (loss)	32.2		19.0		70.1	(549.0)
Investment in unconsolidated affiliates	425.4		356.4		425.4	356.4
Total assets	427.8		360.2		427.8	360.2
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)		(0.5)		(2.1)	(2.1)
Depreciation and amortization	0.0		0.0		0.0	0.0
Taxes other than income	0.7		0.8		2.1	2.4
OPERATING INCOME	0.0		(0.3)		0.0	(0.3)
Equity in earnings of unconsolidated affiliates	0.0		0.0		0.0	0.0	[1]
Other Nonoperating Income (Expense)	(0.5)		1.7		(0.6)	3.0
Interest expense	2.6		1.7		3.8	4.1
Income Tax Expense (Benefit)	1.3		11.7		(3.6)	(45.7)
Net income (loss)	(4.4)		(12.0)		(0.8)	44.3
Investment in unconsolidated affiliates	26.3		20.1		26.3	20.1
Total assets	127.4		109.1		127.4	109.1
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
Equity in earnings of unconsolidated affiliates	0.0		0.0		0.0	0.0	[1]
Other Nonoperating Income (Expense)	(0.6)		(0.2)		(0.8)	(1.2)
Interest expense	(0.6)		(0.2)		(0.8)	(1.2)
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		0.0	0.0
Total assets	$ (115.0)		$ (397.8)		$ (115.0)	$ (397.8)

[1] In 2020, OGE Energy recorded a $780.0 million impairment on its investment in Enable, as further discussed in Note 4