Report of Business Segments (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Segment Reporting Information [Line Items]
Operating revenues	$ 586.4	$ 551.4	$ 1,042.4	$ 984.5
Cost of sales	232.1	197.7	440.8	375.6
Other operation and maintenance	114.8	127.6	238.8	241.5
Depreciation and amortization	74.7	80.1	130.3	158.6
Taxes other than income	21.3	20.1	45.2	45.0
OPERATING INCOME	143.5	125.9	187.3	163.8
Equity in earnings of unconsolidated affiliates	29.4	16.7	65.0	45.0
Other income	15.6	5.5	27.2	11.0
Interest expense	37.1	36.0	72.1	72.3
Income tax expense (benefit)	46.6	40.6	66.6	50.8
Net income	104.8	71.5	140.8	96.7
Investment in unconsolidated affiliates	1,159.1	1,168.8	1,159.1	1,168.8	$ 1,158.6
Total assets	10,420.5	9,659.2	10,420.5	9,659.2	$ 9,939.6
Electric Utility [Member]
Segment Reporting Information [Line Items]
Operating revenues	586.4	551.4	1,042.4	984.5
Cost of sales	232.1	197.7	440.8	375.6
Other operation and maintenance	116.5	124.8	242.6	241.1
Depreciation and amortization	73.7	78.4	128.4	155.1
Taxes other than income	20.2	19.1	42.5	42.7
OPERATING INCOME	143.9	131.4	188.1	170.0
Equity in earnings of unconsolidated affiliates	0.0	0.0	0.0	0.0
Other income	15.6	7.0	28.5	12.3
Interest expense	35.6	35.0	69.2	70.5
Income tax expense (benefit)	37.7	31.1	45.0	33.4
Net income	86.2	72.3	102.4	78.4
Investment in unconsolidated affiliates	0.0	0.0	0.0	0.0
Total assets	9,199.0	8,380.2	9,199.0	8,380.2
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	0.2	7.8	0.3	8.0
Depreciation and amortization	0.0	0.0	0.0	0.0
Taxes other than income	0.3	0.0	0.5	0.0
OPERATING INCOME	(0.5)	(7.8)	(0.8)	(8.0)
Equity in earnings of unconsolidated affiliates	29.4	16.7	65.0	45.0
Other income	0.0	0.0	0.1	0.0
Interest expense	0.0	0.0	0.0	0.0
Income tax expense (benefit)	10.6	9.3	26.0	19.4
Net income	18.3	(0.4)	38.3	17.6
Investment in unconsolidated affiliates	1,153.9	1,168.8	1,153.9	1,168.8
Total assets	1,513.9	1,481.8	1,513.9	1,481.8
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	(1.9)	(5.0)	(4.1)	(7.6)
Depreciation and amortization	1.0	1.7	1.9	3.5
Taxes other than income	0.8	1.0	2.2	2.3
OPERATING INCOME	0.1	2.3	0.0	1.8
Equity in earnings of unconsolidated affiliates	0.0	0.0	0.0	0.0
Other income	0.0	(1.4)	(1.3)	(1.1)
Interest expense	1.5	1.1	3.0	2.0
Income tax expense (benefit)	(1.7)	0.2	(4.4)	(2.0)
Net income	0.3	(0.4)	0.1	0.7
Investment in unconsolidated affiliates	5.2	0.0	5.2	0.0
Total assets	89.2	94.9	89.2	94.9
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	0.0	(0.1)	(0.1)	(0.2)
Interest expense	0.0	(0.1)	(0.1)	(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	(381.6)	(297.7)	(381.6)	(297.7)
OGE Energy [Member]
Segment Reporting Information [Line Items]
Goodwill, Impairment Loss	$ 0.0	$ 0.0	$ 0.0	$ (1.8)