Dispositions, Assets and Liabilities Held for Sale and Impairments (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Total Income on Discontinued Operations as Presented on the Condensed Consolidated Statements of Income	$ 0.0	$ 7.8	$ (2.5)	$ 18.2
Total Assets Classified as Held for Sale on the Balance Sheets	1,915.3		1,915.3
Total Liabilities Classified as Held for Sale on the Balance Sheets	231.0		231.0
Asset Impairments and Other Related Charges	2,264.9	0.0	2,264.9	0.0
Indiana Michigan Power Co [Member]
Asset Impairments and Other Related Charges	10.5	0.0	10.5	0.0
Corporate and Other [Member]
Other Revenues		129.1		372.2
Other Operation Expense		96.7		273.1
Maintenance Expense		4.2		19.9
Depreciation and Amortization Expense		8.8		26.9
Taxes Other Than Income Taxes		2.7		9.9
Total Expense		112.4		329.8
Other Expense		(5.4)		(14.5)
Pretax Income of Discontinued Operations		11.3		27.9
Income Tax Expense		3.6		9.7
Equity Earnings of Unconsolidated Subsidiaries		0.1		0.0
Total Income on Discontinued Operations as Presented on the Condensed Consolidated Statements of Income		7.8		18.2
Vertically Integrated Utilities [Member]
Total Income on Discontinued Operations as Presented on the Condensed Consolidated Statements of Income	0.0	0.0	0.0	0.0
Total Assets Classified as Held for Sale on the Balance Sheets	0.0		0.0
Total Liabilities Classified as Held for Sale on the Balance Sheets	0.0		0.0
Generation And Marketing [Member]
Total Income on Discontinued Operations as Presented on the Condensed Consolidated Statements of Income	0.0	0.0	0.0	0.0
Fuel	139.7		139.7
Materials and Supplies	48.7		48.7
Property, Plant and Equipment - Net	1,726.5		1,726.5
Other Class of Assets That Are Not Major	0.4		0.4
Total Assets Classified as Held for Sale on the Balance Sheets	1,915.3		1,915.3
Long-term Debt	134.8		134.8
Waterford Plant Upgrade Liability	53.1		53.1
Asset Retirement Obligations	36.3		36.3
Other Classes of Liabilities That Are Not Major	6.8		6.8
Total Liabilities Classified as Held for Sale on the Balance Sheets	231.0		231.0
Discontinued Operations and Disposal Groups (Textuals)
Cash Proceeds from Sale of Disposition Plants, Net			2,200.0
Pretax Income (Loss) of Disposal Group	116.0	$ 118.0	312.0	404.0
Generation And Marketing [Member] | I&M Price River Coal Reserves [Member]
Public Utilities, Property, Plant and Equipment, Net	11.0		11.0
Generation And Marketing [Member] | Merchant Coal-Fired Generation Assets [Member]
Public Utilities, Property, Plant and Equipment, Net	2,139.4		2,139.4
Fair Value of Generating Unit	0.0		0.0
Asset Impairments and Other Related Charges			2,139.4
Generation And Marketing [Member] | Trent and Desert Sky Wind Farms [Member]
Public Utilities, Property, Plant and Equipment, Net	118.7		118.7
Fair Value of Generating Unit	46.0		46.0
Asset Impairments and Other Related Charges			72.7
Generation And Marketing [Member] | Coal Reserves [Member]
Public Utilities, Property, Plant and Equipment, Net	56.6		56.6
Fair Value of Generating Unit	3.8		3.8
Asset Impairments and Other Related Charges			52.8
Generation And Marketing [Member] | Total Impaired Assets [Member]
Public Utilities, Property, Plant and Equipment, Net	2,314.7		2,314.7
Fair Value of Generating Unit	$ 49.8		49.8
Asset Impairments and Other Related Charges			2,264.9
Tanners Creek Plant Units 1 Through 4 [Member] | Vertically Integrated Utilities [Member]
Discontinued Operations and Disposal Groups (Textuals)
Payment On Sale Of Property Plant And Equipment			$ 92.0
Muskingum River Plant [Member] | Generation And Marketing [Member]
Discontinued Operations and Disposal Groups (Textuals)
Payment On Sale Of Property Plant And Equipment				48.0
Gain on Sale of Muskingum River Plant				$ 32.0