Report of Business Segments (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Segment Reporting Information [Line Items]
Operating revenues	$ 456.0	$ 433.1
Cost of sales	208.7	177.9
Other operation and maintenance	124.0	113.9
Depreciation and amortization	55.6	78.5
Taxes other than income	23.9	24.9
OPERATING INCOME	43.8	37.9
Equity in earnings of unconsolidated affiliates	35.6	28.3
Other income (expense)	11.6	5.5
Interest expense	35.0	36.3
Income tax expense (benefit)	20.0	10.2
Net income (loss)	36.0	25.2
Investment in unconsolidated affiliates	1,158.9	1,187.4	$ 1,158.6
Total assets	10,134.9	9,523.3	$ 9,939.6
Electric Utility [Member]
Segment Reporting Information [Line Items]
Operating revenues	456.0	433.1
Cost of sales	208.7	177.9
Other operation and maintenance	126.1	116.3
Depreciation and amortization	54.7	76.7
Taxes other than income	22.3	23.6
OPERATING INCOME	44.2	38.6
Equity in earnings of unconsolidated affiliates	0.0	0.0
Other income (expense)	12.9	5.3
Interest expense	33.6	35.5
Income tax expense (benefit)	7.3	2.3
Net income (loss)	16.2	6.1
Investment in unconsolidated affiliates	0.0	0.0
Total assets	8,950.9	8,288.5
Natural Gas Midstream Operations [Member]
Segment Reporting Information [Line Items]
Operating revenues	0.0	0.0
Cost of sales	0.0	0.0
Other operation and maintenance	0.1	0.2
Depreciation and amortization	0.0	0.0
Taxes other than income	0.2	0.0
OPERATING INCOME	(0.3)	(0.2)
Equity in earnings of unconsolidated affiliates	35.6	28.3
Other income (expense)	0.1	0.0
Interest expense	0.0	0.0
Income tax expense (benefit)	15.4	10.1
Net income (loss)	20.0	18.0
Investment in unconsolidated affiliates	1,158.9	1,187.4
Total assets	1,521.2	1,467.9
Other Operations [Member]
Segment Reporting Information [Line Items]
Operating revenues	0.0	0.0
Cost of sales	0.0	0.0
Other operation and maintenance	(2.2)	(2.6)
Depreciation and amortization	0.9	1.8
Taxes other than income	1.4	1.3
OPERATING INCOME	(0.1)	(0.5)
Equity in earnings of unconsolidated affiliates	0.0	0.0
Other income (expense)	(1.3)	0.3
Interest expense	1.5	0.9
Income tax expense (benefit)	(2.7)	(2.2)
Net income (loss)	(0.2)	1.1
Investment in unconsolidated affiliates	0.0	0.0
Total assets	91.1	98.7
Eliminations [Member]
Segment Reporting Information [Line Items]
Operating revenues	0.0	0.0
Cost of sales	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	0.0
Other income (expense)	(0.1)	(0.1)
Interest expense	(0.1)	(0.1)
Income tax expense (benefit)	0.0	0.0
Net income (loss)	0.0	0.0
Investment in unconsolidated affiliates	0.0	0.0
Total assets	(428.3)	(331.8)
OGE Energy [Member]
Segment Reporting Information [Line Items]
Goodwill, Impairment Loss	$ 0.0	$ (1.8)