Report of Business Segments (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Segment Reporting Information [Line Items]
Operating revenues	$ 716.8	$ 743.9	$ 1,759.2	$ 1,728.4
Cost of sales	255.7	269.8	696.5	645.4
Other operation and maintenance	117.7	113.1	356.5	354.6
Depreciation and amortization	76.9	82.2	207.2	240.8
Taxes other than income	23.2	21.5	68.4	66.5
OPERATING INCOME	243.3	257.3	430.6	421.1
Equity in earnings of unconsolidated affiliates	33.6	34.5	98.6	79.5
Other income	24.8	6.3	52.0	17.3
Interest expense	35.9	35.4	108.0	107.7
Income tax expense (benefit)	82.4	79.1	149.0	129.9
Net income	183.4	183.6	324.2	280.3
Investment in unconsolidated affiliates	1,158.1	1,168.0	1,158.1	1,168.0	$ 1,158.6
Total assets	10,463.7	9,784.0	10,463.7	9,784.0	$ 9,939.6
Electric Utility [Member]
Segment Reporting Information [Line Items]
Operating revenues	716.8	743.9	1,759.2	1,728.4
Cost of sales	255.7	269.8	696.5	645.4
Other operation and maintenance	120.2	115.2	362.8	356.3
Depreciation and amortization	76.2	80.8	204.6	235.9
Taxes other than income	21.7	20.9	64.2	63.6
OPERATING INCOME	243.0	257.2	431.1	427.2
Equity in earnings of unconsolidated affiliates	0.0	0.0	0.0	0.0
Other income	24.7	6.0	53.2	18.3
Interest expense	34.5	34.3	103.7	104.8
Income tax expense (benefit)	71.7	69.0	116.7	102.4
Net income	161.5	159.9	263.9	238.3
Investment in unconsolidated affiliates	0.0	0.0	0.0	0.0
Total assets	9,230.9	8,511.5	9,230.9	8,511.5
Natural Gas Midstream Operations [Member]
Segment Reporting Information [Line Items]
Operating revenues	0.0	0.0	0.0	0.0
Cost of sales	0.0	0.0	0.0	0.0
Other operation and maintenance	(0.8)	(0.1)	(0.5)	7.9
Depreciation and amortization	0.0	0.0	0.0	0.0
Taxes other than income	0.5	0.0	1.0	0.0
OPERATING INCOME	0.3	0.1	(0.5)	(7.9)
Equity in earnings of unconsolidated affiliates	33.6	34.5	98.6	79.5
Other income	0.0	0.0	0.1	0.0
Interest expense	0.0	0.0	0.0	0.0
Income tax expense (benefit)	12.6	12.1	38.6	31.5
Net income	21.3	22.5	59.6	40.1
Investment in unconsolidated affiliates	1,152.9	1,168.0	1,152.9	1,168.0
Total assets	1,503.9	1,503.1	1,503.9	1,503.1
Other Operations [Member]
Segment Reporting Information [Line Items]
Operating revenues	0.0	0.0	0.0	0.0
Cost of sales	0.0	0.0	0.0	0.0
Other operation and maintenance	(1.7)	(2.0)	(5.8)	(9.6)
Depreciation and amortization	0.7	1.4	2.6	4.9
Taxes other than income	1.0	0.6	3.2	2.9
OPERATING INCOME	0.0	0.0	0.0	1.8
Equity in earnings of unconsolidated affiliates	0.0	0.0	0.0	0.0
Other income	0.3	0.3	(1.0)	(0.8)
Interest expense	1.6	1.1	4.6	3.1
Income tax expense (benefit)	(1.9)	(2.0)	(6.3)	(4.0)
Net income	0.6	1.2	0.7	1.9
Investment in unconsolidated affiliates	5.2	0.0	5.2	0.0
Total assets	87.1	93.3	87.1	93.3
Eliminations [Member]
Segment Reporting Information [Line Items]
Operating revenues	0.0	0.0	0.0	0.0
Cost of sales	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
Other income	(0.2)	0.0	(0.3)	(0.2)
Interest expense	(0.2)	0.0	(0.3)	(0.2)
Income tax expense (benefit)	0.0	0.0	0.0	0.0
Net income	0.0	0.0	0.0	0.0
Investment in unconsolidated affiliates	0.0	0.0	0.0	0.0
Total assets	(358.2)	(323.9)	(358.2)	(323.9)
OGE Energy [Member]
Segment Reporting Information [Line Items]
Goodwill, Impairment Loss	$ 0.0	$ 0.0	$ 0.0	$ (1.8)