Fair Value Long-term Debt, Other Temporary Investments, Nuclear Trusts (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Book Values and Fair Values of Long - term Debt
Total Long-term Debt Outstanding		$ 19,704.7		$ 19,704.7		$ 19,572.7
Long Term Debt, Fair Value						21,201.3
Other Temporary Investments
Cost		265.7		265.7		375.8
Gross Unrealized Gains		13.5		13.5		11.7
Gross Unrealized Losses		0.0		0.0		(0.7)
Fair Value		279.2		279.2		386.8
Debt and Equity Securities Within Other Temporary Investments
Proceeds from Investment Sales		0.0	$ 0.0	0.0	$ 0.0
Purchases of Investments		0.6	9.5	1.6	10.3
Gross Realized Gains on Investment Sales		0.0	0.0	0.0	0.0
Gross Realized Losses on Investment Sales		0.0	0.0	0.0	0.0
Nuclear Trust Fund Investments
Fair Value		2,230.8		2,230.8		2,106.4
Gross Unrealized Gains		694.4		694.4		611.8
Other-Than-Temporary Impairments		(81.4)		(81.4)		(83.3)
Securities Activity Within Decommissioning and SNF Trusts
Proceeds from Investment Sales		650.0	921.5	2,427.0	1,437.3
Purchases of Investments		656.5	938.4	2,452.9	1,479.1
Gross Realized Gains on Investment Sales		13.9	15.0	41.9	33.8
Gross Realized Losses on Investment Sales		6.5	13.1	22.2	22.8
Contractual Maturities, Fair Value of Debt Securities
Contractual Maturities, Fair Value of Debt Securities		2,230.8		2,230.8		2,106.4
Fair Value Measurements (Textuals)
Adjusted Cost of Debt Securities		913.0		913.0		771.0
Adjusted Cost of Domestic Equity Securities		588.0		588.0		555.0
Includes Debt Included In Liabilities Held For Sale [Member]
Book Values and Fair Values of Long - term Debt
Total Long-term Debt Outstanding	[1],[2]	19,839.5		19,839.5
Long Term Debt, Fair Value		22,840.4		22,840.4
Appalachian Power Co [Member]
Book Values and Fair Values of Long - term Debt
Total Long-term Debt Outstanding		4,033.1		4,033.1		3,930.7
Long Term Debt, Fair Value		4,941.8		4,941.8		4,416.7
Indiana Michigan Power Co [Member]
Book Values and Fair Values of Long - term Debt
Total Long-term Debt Outstanding		2,407.4		2,407.4		2,000.0
Long Term Debt, Fair Value		2,717.8		2,717.8		2,193.6
Nuclear Trust Fund Investments
Fair Value		2,230.8		2,230.8		2,106.4
Gross Unrealized Gains		694.4		694.4		611.8
Other-Than-Temporary Impairments		(81.4)		(81.4)		(83.3)
Securities Activity Within Decommissioning and SNF Trusts
Proceeds from Investment Sales		650.0	921.5	2,427.0	1,437.3
Purchases of Investments		656.5	938.4	2,452.9	1,479.1
Gross Realized Gains on Investment Sales		13.9	15.0	41.9	33.8
Gross Realized Losses on Investment Sales		6.5	$ 13.1	22.2	$ 22.8
Contractual Maturities, Fair Value of Debt Securities
Contractual Maturities, Fair Value of Debt Securities		2,230.8		2,230.8		2,106.4
Fair Value Measurements (Textuals)
Adjusted Cost of Debt Securities		913.0		913.0		771.0
Adjusted Cost of Domestic Equity Securities		588.0		588.0		555.0
Ohio Power Co [Member]
Book Values and Fair Values of Long - term Debt
Total Long-term Debt Outstanding		1,763.4		1,763.4		2,157.7
Long Term Debt, Fair Value		2,213.4		2,213.4		2,472.7
Public Service Co Of Oklahoma [Member]
Book Values and Fair Values of Long - term Debt
Total Long-term Debt Outstanding		1,286.2		1,286.2		1,286.1
Long Term Debt, Fair Value		1,502.6		1,502.6		1,402.9
Southwestern Electric Power Co [Member]
Book Values and Fair Values of Long - term Debt
Total Long-term Debt Outstanding		2,674.0		2,674.0		2,273.5
Long Term Debt, Fair Value		2,943.4		2,943.4		2,417.2
Cash [Member]
Other Temporary Investments
Cost	[3]	159.2		159.2		271.0
Gross Unrealized Gains	[3]	0.0		0.0		0.0
Gross Unrealized Losses	[3]	0.0		0.0		0.0
Fair Value	[3],[4]	159.2		159.2		271.0
Fixed Income Funds [Member]
Nuclear Trust Fund Investments
Fair Value		975.6		975.6		811.2
Gross Unrealized Gains		62.8		62.8		40.2
Other-Than-Temporary Impairments		(3.4)		(3.4)		(4.0)
Contractual Maturities, Fair Value of Debt Securities
Contractual Maturities, Fair Value of Debt Securities		975.6		975.6		811.2
Fixed Income Funds [Member] | Indiana Michigan Power Co [Member]
Nuclear Trust Fund Investments
Fair Value		975.6		975.6		811.2
Gross Unrealized Gains		62.8		62.8		40.2
Other-Than-Temporary Impairments		(3.4)		(3.4)		(4.0)
Contractual Maturities, Fair Value of Debt Securities
Contractual Maturities, Fair Value of Debt Securities		975.6		975.6		811.2
Mutual Funds Fixed Income [Member]
Other Temporary Investments
Cost	[5]	92.3		92.3		91.1
Gross Unrealized Gains	[5]	0.3		0.3		0.0
Gross Unrealized Losses	[5]	0.0		0.0		(0.7)
Fair Value	[5]	92.6		92.6		90.4
Domestic [Member]
Nuclear Trust Fund Investments
Fair Value	[6]	1,220.0		1,220.0		1,126.9
Gross Unrealized Gains		631.6		631.6		571.6
Other-Than-Temporary Impairments		(78.0)		(78.0)		(79.3)
Contractual Maturities, Fair Value of Debt Securities
Contractual Maturities, Fair Value of Debt Securities	[6]	1,220.0		1,220.0		1,126.9
Domestic [Member] | Indiana Michigan Power Co [Member]
Nuclear Trust Fund Investments
Fair Value	[6]	1,220.0		1,220.0		1,126.9
Gross Unrealized Gains		631.6		631.6		571.6
Other-Than-Temporary Impairments		(78.0)		(78.0)		(79.3)
Contractual Maturities, Fair Value of Debt Securities
Contractual Maturities, Fair Value of Debt Securities	[6]	1,220.0		1,220.0		1,126.9
Mutual Funds Equity [Member]
Other Temporary Investments
Cost		14.2		14.2		13.7
Gross Unrealized Gains		13.2		13.2		11.7
Gross Unrealized Losses		0.0		0.0		0.0
Fair Value	[6]	27.4		27.4		25.4
Cash and Cash Equivalents [Member]
Nuclear Trust Fund Investments
Fair Value	[7]	35.2		35.2		168.3
Gross Unrealized Gains		0.0		0.0		0.0
Other-Than-Temporary Impairments		0.0		0.0		0.0
Contractual Maturities, Fair Value of Debt Securities
Contractual Maturities, Fair Value of Debt Securities	[7]	35.2		35.2		168.3
Cash and Cash Equivalents [Member] | Indiana Michigan Power Co [Member]
Nuclear Trust Fund Investments
Fair Value	[7]	35.2		35.2		168.3
Gross Unrealized Gains		0.0		0.0		0.0
Other-Than-Temporary Impairments		0.0		0.0		0.0
Contractual Maturities, Fair Value of Debt Securities
Contractual Maturities, Fair Value of Debt Securities	[7]	35.2		35.2		168.3
US Government Agencies Debt Securities [Member]
Nuclear Trust Fund Investments
Fair Value		892.7		892.7		731.1
Gross Unrealized Gains		55.5		55.5		35.9
Other-Than-Temporary Impairments		(2.1)		(2.1)		(2.6)
Contractual Maturities, Fair Value of Debt Securities
Contractual Maturities, Fair Value of Debt Securities		892.7		892.7		731.1
US Government Agencies Debt Securities [Member] | Indiana Michigan Power Co [Member]
Nuclear Trust Fund Investments
Fair Value		892.7		892.7		731.1
Gross Unrealized Gains		55.5		55.5		35.9
Other-Than-Temporary Impairments		(2.1)		(2.1)		(2.6)
Contractual Maturities, Fair Value of Debt Securities
Contractual Maturities, Fair Value of Debt Securities		892.7		892.7		731.1
Corporate Debt [Member]
Nuclear Trust Fund Investments
Fair Value		66.5		66.5		57.9
Gross Unrealized Gains		6.1		6.1		3.2
Other-Than-Temporary Impairments		(1.0)		(1.0)		(1.1)
Contractual Maturities, Fair Value of Debt Securities
Contractual Maturities, Fair Value of Debt Securities		66.5		66.5		57.9
Corporate Debt [Member] | Indiana Michigan Power Co [Member]
Nuclear Trust Fund Investments
Fair Value		66.5		66.5		57.9
Gross Unrealized Gains		6.1		6.1		3.2
Other-Than-Temporary Impairments		(1.0)		(1.0)		(1.1)
Contractual Maturities, Fair Value of Debt Securities
Contractual Maturities, Fair Value of Debt Securities		66.5		66.5		57.9
State and Local Jurisdiction [Member]
Nuclear Trust Fund Investments
Fair Value		16.4		16.4		22.2
Gross Unrealized Gains		1.2		1.2		1.1
Other-Than-Temporary Impairments		(0.3)		(0.3)		(0.3)
Contractual Maturities, Fair Value of Debt Securities
Contractual Maturities, Fair Value of Debt Securities		16.4		16.4		22.2
State and Local Jurisdiction [Member] | Indiana Michigan Power Co [Member]
Nuclear Trust Fund Investments
Fair Value		16.4		16.4		22.2
Gross Unrealized Gains		1.2		1.2		1.1
Other-Than-Temporary Impairments		(0.3)		(0.3)		(0.3)
Contractual Maturities, Fair Value of Debt Securities
Contractual Maturities, Fair Value of Debt Securities		16.4		16.4		$ 22.2
Within One Year [Member]
Nuclear Trust Fund Investments
Fair Value		330.4		330.4
Contractual Maturities, Fair Value of Debt Securities
Contractual Maturities, Fair Value of Debt Securities		330.4		330.4
Within One Year [Member] | Indiana Michigan Power Co [Member]
Nuclear Trust Fund Investments
Fair Value		330.4		330.4
Contractual Maturities, Fair Value of Debt Securities
Contractual Maturities, Fair Value of Debt Securities		330.4		330.4
One Year To Five Year [Member]
Nuclear Trust Fund Investments
Fair Value		317.3		317.3
Contractual Maturities, Fair Value of Debt Securities
Contractual Maturities, Fair Value of Debt Securities		317.3		317.3
One Year To Five Year [Member] | Indiana Michigan Power Co [Member]
Nuclear Trust Fund Investments
Fair Value		317.3		317.3
Contractual Maturities, Fair Value of Debt Securities
Contractual Maturities, Fair Value of Debt Securities		317.3		317.3
Five Year To Ten Year [Member]
Nuclear Trust Fund Investments
Fair Value		150.4		150.4
Contractual Maturities, Fair Value of Debt Securities
Contractual Maturities, Fair Value of Debt Securities		150.4		150.4
Five Year To Ten Year [Member] | Indiana Michigan Power Co [Member]
Nuclear Trust Fund Investments
Fair Value		150.4		150.4
Contractual Maturities, Fair Value of Debt Securities
Contractual Maturities, Fair Value of Debt Securities		150.4		150.4
After Ten Year [Member]
Nuclear Trust Fund Investments
Fair Value		177.5		177.5
Contractual Maturities, Fair Value of Debt Securities
Contractual Maturities, Fair Value of Debt Securities		177.5		177.5
After Ten Year [Member] | Indiana Michigan Power Co [Member]
Nuclear Trust Fund Investments
Fair Value		177.5		177.5
Contractual Maturities, Fair Value of Debt Securities
Contractual Maturities, Fair Value of Debt Securities		$ 177.5		$ 177.5

[1] Amounts include debt related to the Lawrenceburg Plant that has been classified as Liabilities Held for Sale on the balance sheet. See “Gavin, Waterford, Darby and Lawrenceburg Plants (Generation & Marketing Segment)” section of Note 6 for additional information.

[2] Amounts include debt related to the Lawrenceburg Plant that has been classified as Liabilities Held for Sale on the balance sheet. See “Gavin, Waterford, Darby and Lawrenceburg Plants (Generation & Marketing Segment)” section of Note 6 for additional information.

[3] Primarily represents amounts held for the repayment of debt.

[4] Amounts in “Other’’ column primarily represent cash deposits in bank accounts with financial institutions or with third parties. Level 1 and Level 2 amounts primarily represent investments in money market funds.

[5] Primarily short and intermediate maturities which may be sold and do not contain maturity dates.

[6] Amounts represent publicly traded equity securities and equity-based mutual funds.

[7] Amounts in “Other’’ column primarily represent accrued interest receivables from financial institutions. Level 1 amounts primarily represent investments in money market funds.