Business Segments (Segment Financial Information) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Segment Reporting Information [Line Items]
Gain (Loss) on Disposition of Business	$ (13.6)	$ 0.0
External customer revenues	293.2	323.9
Intersegment revenues	0.0	0.0
Total revenues	293.2	323.9
Depreciation and amortization	20.1	28.0
Fuel Costs	33.8	54.1
Depreciation and amortization	20.1	28.0
Impairment of Long-Lived Assets Held-for-use	0.0	66.4
Interest expense	28.0	27.3
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	20.5	(83.2)
Net income/ (loss)	16.9	(51.7)
Capital expenditures	27.3	41.4
Total assets	1,895.3		$ 2,049.2
Operating Segments [Member] | Transmission and Distribution [Member]
Segment Reporting Information [Line Items]
External customer revenues	195.8	189.8
Intersegment revenues	0.2	0.3
Total revenues	196.0	190.1
Depreciation and amortization	18.6	18.1
Impairment of Long-Lived Assets Held-for-use	0.0	0.0
Interest expense	8.2	7.6
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	19.3	25.0
Capital expenditures	24.6	26.3
Total assets	1,666.9		1,689.4
Operating Segments [Member] | Generation [Member]
Segment Reporting Information [Line Items]
Gain (Loss) on Disposition of Business	52.5
External customer revenues	95.0	131.8
Intersegment revenues	0.0	0.0
Total revenues	95.0	131.8
Depreciation and amortization	1.5	7.0
Impairment of Long-Lived Assets Held-for-use	0.0	66.3
Interest expense	0.0	0.0
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	71.0	(86.8)
Capital expenditures	1.7	14.1
Total assets	228.9		275.0
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Gain (Loss) on Disposition of Business	54.4
External customer revenues	2.4	2.3
Intersegment revenues	0.6	1.4
Total revenues	3.0	3.7
Depreciation and amortization	0.0	2.9
Impairment of Long-Lived Assets Held-for-use	0.0	0.1
Interest expense	19.8	19.8
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	(69.8)	(21.4)
Capital expenditures	1.0	1.0
Total assets	580.4		468.0
Adjustments and Eliminations [Member]
Segment Reporting Information [Line Items]
External customer revenues	0.0	0.0
Intersegment revenues	(0.8)	(1.7)
Total revenues	(0.8)	(1.7)
Depreciation and amortization	0.0	0.0
Impairment of Long-Lived Assets Held-for-use	0.0	0.0
Interest expense	0.0	(0.1)
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	0.0	0.0
Capital expenditures	0.0	0.0
Total assets	(580.9)		(383.2)
Subsidiaries [Member]
Segment Reporting Information [Line Items]
Gain (Loss) on Disposition of Business	(12.4)	0.0
Depreciation and amortization	18.6	18.1
Fuel Costs	0.9	0.0
Depreciation and amortization	18.6	23.5
Impairment of Long-Lived Assets Held-for-use	0.0	66.3
Interest expense	8.2	7.6
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	19.3	25.0
Net income / (loss) from continuing operations	15.7	17.0
Discontinued operations, net of tax	0.0	(58.8)
Net income/ (loss)	15.7	(41.8)
Total assets	1,666.9		$ 1,689.4
Subsidiaries [Member] | Generation [Member]
Segment Reporting Information [Line Items]
Discontinued operations, net of tax		(58.8)
Other Operating Income (Expense) [Member]
Segment Reporting Information [Line Items]
Gain (Loss) on Disposition of Business	13.6	0.0
AES Ohio Generation peakers [Member]
Segment Reporting Information [Line Items]
Gain (Loss) on Disposition of Business	1.9
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	$ 7.2	$ 0.3