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Fair Value Long-term Debt, Other Temporary Investments, Nuclear Trusts (Details) - USD ($)
$ in Millions
3 Months Ended 12 Months Ended
Mar. 31, 2020
Mar. 31, 2019
Dec. 31, 2019
Book Values and Fair Values of Long - term Debt      
Total Long-term Debt Outstanding $ 27,892.7   $ 26,725.5
Long Term Debt, Fair Value [1] 29,776.6   30,172.0
Other Temporary Investments      
Cost 289.8   367.1
Gross Unrealized Gains 11.6   21.4
Gross Unrealized Losses 0.0   0.0
Other Short-term Investments 301.4   388.5
Debt and Equity Securities Within Other Temporary Investments      
Proceeds from Investment Sales 23.2 $ 0.0  
Purchases of Investments 6.7 0.1  
Gross Realized Gains on Investment Sales 2.0 0.0  
Gross Realized Losses on Investment Sales 0.1 0.0  
Nuclear Trust Fund Investments      
Fair Value 2,679.2   2,975.7
Contractual Maturities, Fair Value of Debt Securities      
Fair Value 2,679.2   2,975.7
Spent Nuclear Fuel and Decommissioning Trusts [Member]      
Unrealized Gain on Securities 801.0   1,100.0
Nuclear Trust Fund Investments      
Gross Unrealized Gains 906.5   1,205.9
Other-Than-Temporary Impairments (7.4)   (7.9)
Securities Activity Within Decommissioning and SNF Trusts      
Proceeds from Investment Sales 612.4 111.9  
Purchases of Investments 626.0 130.3  
Gross Realized Gains on Investment Sales 10.9 12.3  
Gross Realized Losses on Investment Sales 17.0 13.8  
Contractual Maturities, Fair Value of Debt Securities      
Within 1 year 238.9    
After 1 year through 5 years 404.5    
After 5 years through 10 years 282.5    
After 10 years 312.6    
Fair Value Measurements (Textuals)      
Adjusted Cost of Debt Securities 1,100.0   1,100.0
Adjusted Cost of Domestic Equity Securities 616.0   623.0
Unrealized Loss on Securities 23.0   5.0
Cash [Member]      
Other Temporary Investments      
Cost [2] 151.9   214.7
Gross Unrealized Gains [2] 0.0   0.0
Gross Unrealized Losses [2] 0.0   0.0
Other Short-term Investments [2],[3] 151.9   214.7
Fixed Income Funds [Member]      
Nuclear Trust Fund Investments      
Fair Value 1,238.5   1,192.5
Contractual Maturities, Fair Value of Debt Securities      
Fair Value 1,238.5   1,192.5
Fixed Income Funds [Member] | Spent Nuclear Fuel and Decommissioning Trusts [Member]      
Nuclear Trust Fund Investments      
Gross Unrealized Gains 128.9   61.5
Other-Than-Temporary Impairments (7.4)   (7.9)
Mutual Funds Fixed Income [Member]      
Other Temporary Investments      
Cost [4] 118.6   123.2
Gross Unrealized Gains [4] 0.4   0.1
Gross Unrealized Losses [4] 0.0   0.0
Other Short-term Investments [4] 119.0   123.3
Domestic [Member]      
Nuclear Trust Fund Investments      
Fair Value [5] 1,393.8   1,767.9
Contractual Maturities, Fair Value of Debt Securities      
Fair Value [5] 1,393.8   1,767.9
Domestic [Member] | Spent Nuclear Fuel and Decommissioning Trusts [Member]      
Nuclear Trust Fund Investments      
Gross Unrealized Gains [6] 777.6   1,144.4
Other-Than-Temporary Impairments 0.0   0.0
Mutual Funds Equity [Member]      
Other Temporary Investments      
Cost 19.3   29.2
Gross Unrealized Gains 11.2   21.3
Gross Unrealized Losses 0.0   0.0
Other Short-term Investments [5] 30.5   50.5
Cash and Cash Equivalents [Member]      
Nuclear Trust Fund Investments      
Fair Value [7] 46.9   15.3
Contractual Maturities, Fair Value of Debt Securities      
Fair Value [7] 46.9   15.3
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
US Government Agencies Debt Securities [Member]      
Nuclear Trust Fund Investments      
Fair Value 1,026.1   1,112.5
Contractual Maturities, Fair Value of Debt Securities      
Fair Value 1,026.1   1,112.5
US Government Agencies Debt Securities [Member] | Spent Nuclear Fuel and Decommissioning Trusts [Member]      
Nuclear Trust Fund Investments      
Gross Unrealized Gains 121.4   55.5
Other-Than-Temporary Impairments (5.6)   (6.1)
Corporate Debt [Member]      
Nuclear Trust Fund Investments      
Fair Value 62.7   72.4
Contractual Maturities, Fair Value of Debt Securities      
Fair Value 62.7   72.4
Corporate Debt [Member] | Spent Nuclear Fuel and Decommissioning Trusts [Member]      
Nuclear Trust Fund Investments      
Gross Unrealized Gains 6.0   5.3
Other-Than-Temporary Impairments (1.6)   (1.6)
State and Local Jurisdiction [Member]      
Nuclear Trust Fund Investments      
Fair Value 149.7   7.6
Contractual Maturities, Fair Value of Debt Securities      
Fair Value 149.7   7.6
State and Local Jurisdiction [Member] | Spent Nuclear Fuel and Decommissioning Trusts [Member]      
Nuclear Trust Fund Investments      
Gross Unrealized Gains 1.5   0.7
Other-Than-Temporary Impairments (0.2)   (0.2)
AEP Texas Inc. [Member]      
Book Values and Fair Values of Long - term Debt      
Total Long-term Debt Outstanding 4,445.4   4,558.4
Long Term Debt, Fair Value 4,637.3   4,981.5
AEP Transmission Co [Member]      
Book Values and Fair Values of Long - term Debt      
Total Long-term Debt Outstanding 3,427.8   3,427.3
Long Term Debt, Fair Value 3,680.7   3,868.0
Appalachian Power Co [Member]      
Book Values and Fair Values of Long - term Debt      
Total Long-term Debt Outstanding 4,352.4   4,363.8
Long Term Debt, Fair Value 4,959.0   5,253.1
Indiana Michigan Power Co [Member]      
Book Values and Fair Values of Long - term Debt      
Total Long-term Debt Outstanding 3,028.0   3,050.2
Long Term Debt, Fair Value 3,318.2   3,453.8
Nuclear Trust Fund Investments      
Fair Value 2,679.2   2,975.7
Contractual Maturities, Fair Value of Debt Securities      
Fair Value 2,679.2   2,975.7
Indiana Michigan Power Co [Member] | Spent Nuclear Fuel and Decommissioning Trusts [Member]      
Unrealized Gain on Securities 801.0   1,149.0
Nuclear Trust Fund Investments      
Gross Unrealized Gains 906.5   1,205.9
Other-Than-Temporary Impairments (7.4)   (7.9)
Securities Activity Within Decommissioning and SNF Trusts      
Proceeds from Investment Sales 612.4 111.9  
Purchases of Investments 626.0 130.3  
Gross Realized Gains on Investment Sales 10.9 12.3  
Gross Realized Losses on Investment Sales 17.0 $ 13.8  
Contractual Maturities, Fair Value of Debt Securities      
Within 1 year 238.9    
After 1 year through 5 years 404.5    
After 5 years through 10 years 282.5    
After 10 years 312.6    
Fair Value Measurements (Textuals)      
Adjusted Cost of Debt Securities 1,000.0   1,000.0
Adjusted Cost of Domestic Equity Securities 616.0   623.0
Unrealized Loss on Securities 23.0   5.0
Indiana Michigan Power Co [Member] | Fixed Income Funds [Member]      
Nuclear Trust Fund Investments      
Fair Value 1,238.5   1,192.5
Contractual Maturities, Fair Value of Debt Securities      
Fair Value 1,238.5   1,192.5
Indiana Michigan Power Co [Member] | Fixed Income Funds [Member] | Spent Nuclear Fuel and Decommissioning Trusts [Member]      
Nuclear Trust Fund Investments      
Gross Unrealized Gains 128.9   61.5
Other-Than-Temporary Impairments (7.4)   (7.9)
Indiana Michigan Power Co [Member] | Domestic [Member]      
Nuclear Trust Fund Investments      
Fair Value [5] 1,393.8   1,767.9
Contractual Maturities, Fair Value of Debt Securities      
Fair Value [5] 1,393.8   1,767.9
Indiana Michigan Power Co [Member] | Domestic [Member] | Spent Nuclear Fuel and Decommissioning Trusts [Member]      
Nuclear Trust Fund Investments      
Gross Unrealized Gains [6] 777.6   1,144.4
Other-Than-Temporary Impairments [6] 0.0   0.0
Indiana Michigan Power Co [Member] | Cash and Cash Equivalents [Member]      
Nuclear Trust Fund Investments      
Fair Value [7] 46.9   15.3
Contractual Maturities, Fair Value of Debt Securities      
Fair Value [7] 46.9   15.3
Indiana Michigan Power Co [Member] | 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
Indiana Michigan Power Co [Member] | US Government Agencies Debt Securities [Member]      
Nuclear Trust Fund Investments      
Fair Value 1,026.1   1,112.5
Contractual Maturities, Fair Value of Debt Securities      
Fair Value 1,026.1   1,112.5
Indiana Michigan Power Co [Member] | US Government Agencies Debt Securities [Member] | Spent Nuclear Fuel and Decommissioning Trusts [Member]      
Nuclear Trust Fund Investments      
Gross Unrealized Gains 121.4   55.5
Other-Than-Temporary Impairments (5.6)   (6.1)
Indiana Michigan Power Co [Member] | Corporate Debt [Member]      
Nuclear Trust Fund Investments      
Fair Value 62.7   72.4
Contractual Maturities, Fair Value of Debt Securities      
Fair Value 62.7   72.4
Indiana Michigan Power Co [Member] | Corporate Debt [Member] | Spent Nuclear Fuel and Decommissioning Trusts [Member]      
Nuclear Trust Fund Investments      
Gross Unrealized Gains 6.0   5.3
Other-Than-Temporary Impairments (1.6)   (1.6)
Indiana Michigan Power Co [Member] | State and Local Jurisdiction [Member]      
Nuclear Trust Fund Investments      
Fair Value 149.7   7.6
Contractual Maturities, Fair Value of Debt Securities      
Fair Value 149.7   7.6
Indiana Michigan Power Co [Member] | State and Local Jurisdiction [Member] | Spent Nuclear Fuel and Decommissioning Trusts [Member]      
Nuclear Trust Fund Investments      
Gross Unrealized Gains 1.5   0.7
Other-Than-Temporary Impairments (0.2)   (0.2)
Ohio Power Co [Member]      
Book Values and Fair Values of Long - term Debt      
Total Long-term Debt Outstanding 2,429.1   2,082.0
Long Term Debt, Fair Value 2,795.3   2,554.3
Public Service Co Of Oklahoma [Member]      
Book Values and Fair Values of Long - term Debt      
Total Long-term Debt Outstanding 1,386.3   1,386.2
Long Term Debt, Fair Value 1,553.9   1,603.3
Southwestern Electric Power Co [Member]      
Book Values and Fair Values of Long - term Debt      
Total Long-term Debt Outstanding 2,654.4   2,655.6
Long Term Debt, Fair Value 2,776.5   2,927.9
Equity Units [Member]      
Book Values and Fair Values of Long - term Debt      
Long Term Debt, Fair Value $ 777.0   $ 871.0
[1]
The fair value amount includes debt related to AEP’s Equity Units issued in March 2019 and has a fair value of $777 million and $871 million as of March 31, 2020 and December 31, 2019, respectively. See “Equity Units” section of Note 12 for additional information.
[2]
Primarily represents amounts held for the repayment of debt.
[3]
Amounts in “Other’’ column primarily represent cash deposits in bank accounts with financial institutions or third-parties.  Level 1 and Level 2 amounts primarily represent investments in money market funds.
[4]
Primarily short and intermediate maturities which may be sold and do not contain maturity dates.
[5]
Amounts represent publicly traded equity securities and equity-based mutual funds.
[6]
Amount reported as Gross Unrealized Gains includes unrealized gains of $801 million and $1.1 billion and unrealized losses of $23 million and $5 million as of March 31, 2020 and December 31, 2019, respectively.
[7]
Amounts in “Other’’ column primarily represent accrued interest receivables from financial institutions.  Level 1 amounts primarily represent investments in money market funds.