Fair Value Long-term Debt, Other Temporary Investments, Nuclear Trusts (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended			12 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018		Sep. 30, 2017	Dec. 31, 2017
Book Values and Fair Values of Long - term Debt
Total Long-term Debt Outstanding		$ 22,774.0		$ 22,774.0			$ 21,173.3
Long Term Debt, Fair Value		23,869.1		23,869.1			23,649.6
Other Temporary Investments
Cost		293.9		293.9			341.4
Gross Unrealized Gains		22.2		22.2			19.7
Gross Unrealized Losses		(2.8)		(2.8)			(1.4)
Other Short-term Investments		313.3		313.3			359.7
Debt and Equity Securities Within Other Temporary Investments
Proceeds from Investment Sales		0.0	$ 0.0	0.0		$ 0.0
Purchases of Investments		0.8	12.6	2.2		13.6
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		2,666.0		2,666.0			2,527.6
Contractual Maturities, Fair Value of Debt Securities
Fair Value		2,666.0		2,666.0			2,527.6
Fair Value Measurements (Textuals)
Long Term Debt, Fair Value		23,869.1		23,869.1			23,649.6
Spent Nuclear Fuel and Decommissioning Trusts [Member]
Nuclear Trust Fund Investments
Gross Unrealized Gains				1,004.3			902.5
Other-Than-Temporary Impairments				(8.0)			(80.5)
Securities Activity Within Decommissioning and SNF Trusts
Proceeds from Investment Sales		513.1	519.5	1,550.9		1,808.6
Purchases of Investments		521.2	525.0	1,589.0		1,842.2
Gross Realized Gains on Investment Sales		3.9	9.8	27.7		198.1
Gross Realized Losses on Investment Sales		3.5	5.2	22.2		145.4
Contractual Maturities, Fair Value of Debt Securities
Within 1 year		348.7		348.7
After 1 year through 5 years		331.4		331.4
After 5 years through 10 years		177.0		177.0
After 10 years		157.7		157.7
Fair Value Measurements (Textuals)
Adjusted Cost of Debt Securities		1,000.0		1,000.0			1,000.0
Adjusted Cost of Domestic Equity Securities		619.0		619.0			594.0
AEP Texas Inc. [Member]
Book Values and Fair Values of Long - term Debt
Total Long-term Debt Outstanding		3,914.4		3,914.4			3,649.3
Long Term Debt, Fair Value		4,019.2		4,019.2			3,964.8
Fair Value Measurements (Textuals)
Long Term Debt, Fair Value		4,019.2		4,019.2			3,964.8
AEP Transmission Co [Member]
Book Values and Fair Values of Long - term Debt
Total Long-term Debt Outstanding		2,872.6		2,872.6			2,550.4
Long Term Debt, Fair Value		2,861.1		2,861.1			2,782.9
Fair Value Measurements (Textuals)
Long Term Debt, Fair Value		2,861.1		2,861.1			2,782.9
Appalachian Power Co [Member]
Book Values and Fair Values of Long - term Debt
Total Long-term Debt Outstanding		4,061.7		4,061.7			3,980.1
Long Term Debt, Fair Value		4,629.8		4,629.8			4,782.6
Fair Value Measurements (Textuals)
Long Term Debt, Fair Value		4,629.8		4,629.8			4,782.6
Indiana Michigan Power Co [Member]
Book Values and Fair Values of Long - term Debt
Total Long-term Debt Outstanding		3,062.4		3,062.4			2,745.1
Long Term Debt, Fair Value		3,161.9		3,161.9			3,014.7
Nuclear Trust Fund Investments
Fair Value		2,666.0		2,666.0			2,527.6
Contractual Maturities, Fair Value of Debt Securities
Fair Value		2,666.0		2,666.0			2,527.6
Fair Value Measurements (Textuals)
Long Term Debt, Fair Value		3,161.9		3,161.9			3,014.7
Indiana Michigan Power Co [Member] | Spent Nuclear Fuel and Decommissioning Trusts [Member]
Nuclear Trust Fund Investments
Gross Unrealized Gains				1,004.3			902.5
Other-Than-Temporary Impairments				(8.0)			(80.5)
Securities Activity Within Decommissioning and SNF Trusts
Proceeds from Investment Sales		513.1	519.5	1,550.9		1,808.6
Purchases of Investments		521.2	525.0	1,589.0		1,842.2
Gross Realized Gains on Investment Sales		3.9	9.8	27.7		198.1
Gross Realized Losses on Investment Sales		3.5	$ 5.2	22.2		$ 145.4
Contractual Maturities, Fair Value of Debt Securities
Within 1 year		348.7		348.7
After 1 year through 5 years		331.4		331.4
After 5 years through 10 years		177.0		177.0
After 10 years		157.7		157.7
Fair Value Measurements (Textuals)
Adjusted Cost of Debt Securities		1,000.0		1,000.0			1,000.0
Adjusted Cost of Domestic Equity Securities		619.0		619.0			594.0
Ohio Power Co [Member]
Book Values and Fair Values of Long - term Debt
Total Long-term Debt Outstanding		1,716.3		1,716.3			1,719.3
Long Term Debt, Fair Value		1,941.9		1,941.9			2,064.3
Fair Value Measurements (Textuals)
Long Term Debt, Fair Value		1,941.9		1,941.9			2,064.3
Public Service Co Of Oklahoma [Member]
Book Values and Fair Values of Long - term Debt
Total Long-term Debt Outstanding		1,286.9		1,286.9			1,286.5
Long Term Debt, Fair Value		1,373.4		1,373.4			1,457.1
Fair Value Measurements (Textuals)
Long Term Debt, Fair Value		1,373.4		1,373.4			1,457.1
Southwestern Electric Power Co [Member]
Book Values and Fair Values of Long - term Debt
Total Long-term Debt Outstanding		3,072.7		3,072.7			2,441.9
Long Term Debt, Fair Value		3,068.4		3,068.4			2,645.9
Fair Value Measurements (Textuals)
Long Term Debt, Fair Value		3,068.4		3,068.4			2,645.9
Cash [Member]
Other Temporary Investments
Cost	[1]	170.4		170.4			220.1
Gross Unrealized Gains	[1]	0.0		0.0			0.0
Gross Unrealized Losses	[1]	0.0		0.0			0.0
Other Short-term Investments	[1],[2]	170.4		170.4			220.1
Fixed Income Funds [Member]
Nuclear Trust Fund Investments
Fair Value		1,014.8		1,014.8			1,048.7
Contractual Maturities, Fair Value of Debt Securities
Fair Value		1,014.8		1,014.8			1,048.7
Fixed Income Funds [Member] | Spent Nuclear Fuel and Decommissioning Trusts [Member]
Nuclear Trust Fund Investments
Gross Unrealized Gains				13.8			34.3
Other-Than-Temporary Impairments				(8.0)			(5.0)
Fixed Income Funds [Member] | Indiana Michigan Power Co [Member]
Nuclear Trust Fund Investments
Fair Value		1,014.8		1,014.8			1,048.7
Contractual Maturities, Fair Value of Debt Securities
Fair Value		1,014.8		1,014.8			1,048.7
Fixed Income Funds [Member] | Indiana Michigan Power Co [Member] | Spent Nuclear Fuel and Decommissioning Trusts [Member]
Nuclear Trust Fund Investments
Gross Unrealized Gains				13.8			34.3
Other-Than-Temporary Impairments				(8.0)			(5.0)
Mutual Funds Fixed Income [Member]
Other Temporary Investments
Cost	[3]	105.9		105.9			104.3
Gross Unrealized Gains	[3]	0.0		0.0			0.0
Gross Unrealized Losses	[3]	(2.8)		(2.8)			(1.4)
Other Short-term Investments	[3]	103.1		103.1			102.9
Domestic [Member]
Nuclear Trust Fund Investments
Fair Value	[4]	1,609.6		1,609.6			1,461.7
Contractual Maturities, Fair Value of Debt Securities
Fair Value	[4]	1,609.6		1,609.6			1,461.7
Domestic [Member] | Spent Nuclear Fuel and Decommissioning Trusts [Member]
Unrealized Gain on Securities				995.8
Nuclear Trust Fund Investments
Gross Unrealized Gains				990.5			868.2
Other-Than-Temporary Impairments				0.0	[5]		(75.5)
Fair Value Measurements (Textuals)
Unrealized Loss on Securities				5.3
Domestic [Member] | Indiana Michigan Power Co [Member]
Nuclear Trust Fund Investments
Fair Value	[4]	1,609.6		1,609.6			1,461.7
Contractual Maturities, Fair Value of Debt Securities
Fair Value	[4]	1,609.6		1,609.6			1,461.7
Domestic [Member] | Indiana Michigan Power Co [Member] | Spent Nuclear Fuel and Decommissioning Trusts [Member]
Nuclear Trust Fund Investments
Gross Unrealized Gains				990.5			868.2
Other-Than-Temporary Impairments				0.0	[5]		(75.5)
Mutual Funds Equity [Member]
Other Temporary Investments
Cost		17.6		17.6			17.0
Gross Unrealized Gains		22.2		22.2			19.7
Gross Unrealized Losses		0.0		0.0			0.0
Other Short-term Investments	[4]	39.8		39.8			36.7
Cash and Cash Equivalents [Member]
Nuclear Trust Fund Investments
Fair Value	[6]	41.6		41.6			17.2
Contractual Maturities, Fair Value of Debt Securities
Fair Value	[6]	41.6		41.6			17.2
Cash and Cash Equivalents [Member] | Spent Nuclear Fuel and Decommissioning Trusts [Member]
Nuclear Trust Fund Investments
Gross Unrealized Gains				0.0			0.0
Other-Than-Temporary Impairments				0.0			0.0
Cash and Cash Equivalents [Member] | Indiana Michigan Power Co [Member]
Nuclear Trust Fund Investments
Fair Value	[6]	41.6		41.6			17.2
Contractual Maturities, Fair Value of Debt Securities
Fair Value	[6]	41.6		41.6			17.2
Cash and Cash Equivalents [Member] | Indiana Michigan Power Co [Member] | Spent Nuclear Fuel and Decommissioning Trusts [Member]
Nuclear Trust Fund Investments
Gross Unrealized Gains				0.0			0.0
Other-Than-Temporary Impairments				0.0			0.0
US Government Agencies Debt Securities [Member]
Nuclear Trust Fund Investments
Fair Value		951.9		951.9			981.2
Contractual Maturities, Fair Value of Debt Securities
Fair Value		951.9		951.9			981.2
US Government Agencies Debt Securities [Member] | Spent Nuclear Fuel and Decommissioning Trusts [Member]
Nuclear Trust Fund Investments
Gross Unrealized Gains				12.2			29.7
Other-Than-Temporary Impairments				(6.2)			(3.6)
US Government Agencies Debt Securities [Member] | Indiana Michigan Power Co [Member]
Nuclear Trust Fund Investments
Fair Value		951.9		951.9			981.2
Contractual Maturities, Fair Value of Debt Securities
Fair Value		951.9		951.9			981.2
US Government Agencies Debt Securities [Member] | Indiana Michigan Power Co [Member] | Spent Nuclear Fuel and Decommissioning Trusts [Member]
Nuclear Trust Fund Investments
Gross Unrealized Gains				12.2			29.7
Other-Than-Temporary Impairments				(6.2)			(3.6)
Corporate Debt [Member]
Nuclear Trust Fund Investments
Fair Value		54.4		54.4			58.7
Contractual Maturities, Fair Value of Debt Securities
Fair Value		54.4		54.4			58.7
Corporate Debt [Member] | Spent Nuclear Fuel and Decommissioning Trusts [Member]
Nuclear Trust Fund Investments
Gross Unrealized Gains				1.1			3.8
Other-Than-Temporary Impairments				(1.6)			(1.2)
Corporate Debt [Member] | Indiana Michigan Power Co [Member]
Nuclear Trust Fund Investments
Fair Value		54.4		54.4			58.7
Contractual Maturities, Fair Value of Debt Securities
Fair Value		54.4		54.4			58.7
Corporate Debt [Member] | Indiana Michigan Power Co [Member] | Spent Nuclear Fuel and Decommissioning Trusts [Member]
Nuclear Trust Fund Investments
Gross Unrealized Gains				1.1			3.8
Other-Than-Temporary Impairments				(1.6)			(1.2)
State and Local Jurisdiction [Member]
Nuclear Trust Fund Investments
Fair Value		8.5		8.5			8.8
Contractual Maturities, Fair Value of Debt Securities
Fair Value		8.5		8.5			8.8
State and Local Jurisdiction [Member] | Spent Nuclear Fuel and Decommissioning Trusts [Member]
Nuclear Trust Fund Investments
Gross Unrealized Gains				0.5			0.8
Other-Than-Temporary Impairments				(0.2)			(0.2)
State and Local Jurisdiction [Member] | Indiana Michigan Power Co [Member]
Nuclear Trust Fund Investments
Fair Value		8.5		8.5			8.8
Contractual Maturities, Fair Value of Debt Securities
Fair Value		$ 8.5		8.5			8.8
State and Local Jurisdiction [Member] | Indiana Michigan Power Co [Member] | Spent Nuclear Fuel and Decommissioning Trusts [Member]
Nuclear Trust Fund Investments
Gross Unrealized Gains				0.5			0.8
Other-Than-Temporary Impairments				$ (0.2)			$ (0.2)

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

[2] 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.

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

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

[5] Amount reported as Gross Unrealized Gains includes unrealized gains of $995.8 million and unrealized losses of $5.3 million. AEP adopted ASU 2016-01 during the first quarter of 2018 by means of a modified retrospective approach. Due to the adoption of the ASU, Other-Than-Temporary Impairments are no longer applicable to Equity Securities with readily determinable fair values.

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