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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, 2019
Mar. 31, 2018
Dec. 31, 2018
Book Values and Fair Values of Long - term Debt      
Total Long-term Debt Outstanding $ 24,426.7   $ 23,346.7
Long Term Debt, Fair Value 26,225.6 [1]   24,093.9
Other Temporary Investments      
Cost 289.3   355.0
Gross Unrealized Gains 16.3   16.4
Gross Unrealized Losses (1.3)   (2.3)
Fair Value 304.3   369.1
Debt and Equity Securities Within Other Temporary Investments      
Proceeds from Investment Sales 0.0 $ 0.0  
Purchases of Investments 0.1 0.6  
Gross Realized Gains on Investment Sales 0.0 0.0  
Gross Realized Losses on Investment Sales 0.0 0.0  
Nuclear Trust Fund Investments      
Fair Value 2,684.0   2,474.9
Contractual Maturities, Fair Value of Debt Securities      
Fair Value 2,684.0   2,474.9
Spent Nuclear Fuel and Decommissioning Trusts [Member]      
Nuclear Trust Fund Investments      
Gross Unrealized Gains 993.2   794.7
Other-Than-Temporary Impairments (8.7)   (9.2)
Securities Activity Within Decommissioning and SNF Trusts      
Proceeds from Investment Sales 111.9 508.6  
Purchases of Investments 130.3 525.3  
Gross Realized Gains on Investment Sales 12.3 12.0  
Gross Realized Losses on Investment Sales 13.8 10.9  
Contractual Maturities, Fair Value of Debt Securities      
Within 1 year 356.4    
After 1 year through 5 years 375.4    
After 5 years through 10 years 172.8    
After 10 years 173.9    
Fair Value Measurements (Textuals)      
Adjusted Cost of Debt Securities 1,000.0   1,000.0
Adjusted Cost of Domestic Equity Securities 643.0   629.0
AEP Texas Inc. [Member]      
Book Values and Fair Values of Long - term Debt      
Total Long-term Debt Outstanding 3,778.3   3,881.3
Long Term Debt, Fair Value 3,973.4   3,964.6
AEP Transmission Co [Member]      
Book Values and Fair Values of Long - term Debt      
Total Long-term Debt Outstanding 2,823.4   2,823.0
Long Term Debt, Fair Value 2,949.6   2,782.4
Appalachian Power Co [Member]      
Book Values and Fair Values of Long - term Debt      
Total Long-term Debt Outstanding 4,444.0   4,062.6
Long Term Debt, Fair Value 5,040.6   4,473.3
Indiana Michigan Power Co [Member]      
Book Values and Fair Values of Long - term Debt      
Total Long-term Debt Outstanding 3,011.4   3,035.4
Long Term Debt, Fair Value 3,201.8   3,070.2
Nuclear Trust Fund Investments      
Fair Value 2,684.0   2,474.9
Contractual Maturities, Fair Value of Debt Securities      
Fair Value 2,684.0   2,474.9
Indiana Michigan Power Co [Member] | Spent Nuclear Fuel and Decommissioning Trusts [Member]      
Nuclear Trust Fund Investments      
Gross Unrealized Gains 993.2   794.7
Other-Than-Temporary Impairments (8.7)   (9.2)
Securities Activity Within Decommissioning and SNF Trusts      
Proceeds from Investment Sales 111.9 508.6  
Purchases of Investments 130.3 525.3  
Gross Realized Gains on Investment Sales 12.3 12.0  
Gross Realized Losses on Investment Sales 13.8 10.9  
Contractual Maturities, Fair Value of Debt Securities      
Within 1 year 356.4    
After 1 year through 5 years 375.4    
After 5 years through 10 years 172.8    
After 10 years 173.9    
Fair Value Measurements (Textuals)      
Adjusted Cost of Debt Securities 1,000.0   1,000.0
Adjusted Cost of Domestic Equity Securities 643.0   629.0
Ohio Power Co [Member]      
Book Values and Fair Values of Long - term Debt      
Total Long-term Debt Outstanding 1,693.6   1,716.6
Long Term Debt, Fair Value 1,989.2   1,919.7
Public Service Co Of Oklahoma [Member]      
Book Values and Fair Values of Long - term Debt      
Total Long-term Debt Outstanding 1,386.2   1,287.0
Long Term Debt, Fair Value 1,503.6   1,361.9
Southwestern Electric Power Co [Member]      
Book Values and Fair Values of Long - term Debt      
Total Long-term Debt Outstanding 2,659.1   2,713.4
Long Term Debt, Fair Value 2,763.0   2,670.2
Cash [Member]      
Other Temporary Investments      
Cost [2] 159.6   230.6
Gross Unrealized Gains [2] 0.0   0.0
Gross Unrealized Losses [2] 0.0   0.0
Fair Value [2],[3] 159.6   230.6
Fixed Income Funds [Member]      
Nuclear Trust Fund Investments      
Fair Value 1,078.5   1,057.1
Contractual Maturities, Fair Value of Debt Securities      
Fair Value 1,078.5   1,057.1
Fixed Income Funds [Member] | Spent Nuclear Fuel and Decommissioning Trusts [Member]      
Nuclear Trust Fund Investments      
Gross Unrealized Gains 47.6   28.4
Other-Than-Temporary Impairments (8.7)   (9.2)
Fixed Income Funds [Member] | Indiana Michigan Power Co [Member]      
Nuclear Trust Fund Investments      
Fair Value 1,078.5   1,057.1
Contractual Maturities, Fair Value of Debt Securities      
Fair Value 1,078.5   1,057.1
Fixed Income Funds [Member] | Indiana Michigan Power Co [Member] | Spent Nuclear Fuel and Decommissioning Trusts [Member]      
Nuclear Trust Fund Investments      
Gross Unrealized Gains 47.6   28.4
Other-Than-Temporary Impairments (8.7)   (9.2)
Mutual Funds Fixed Income [Member]      
Other Temporary Investments      
Cost [4] 107.2   106.6
Gross Unrealized Gains [4] 0.0   0.0
Gross Unrealized Losses [4] (1.3)   (2.3)
Fair Value [4] 105.9   104.3
Domestic [Member]      
Nuclear Trust Fund Investments      
Fair Value [5] 1,588.1   1,395.3
Contractual Maturities, Fair Value of Debt Securities      
Fair Value [5] 1,588.1   1,395.3
Domestic [Member] | Spent Nuclear Fuel and Decommissioning Trusts [Member]      
Nuclear Trust Fund Investments      
Gross Unrealized Gains [6] 945.6   766.3
Other-Than-Temporary Impairments [6] 0.0   0.0
Unrealized Gain on Securities 955.0 784.0  
Unrealized Loss on Securities 9.0 18.0  
Domestic [Member] | Indiana Michigan Power Co [Member]      
Nuclear Trust Fund Investments      
Fair Value [5] 1,588.1   1,395.3
Contractual Maturities, Fair Value of Debt Securities      
Fair Value [5] 1,588.1   1,395.3
Domestic [Member] | Indiana Michigan Power Co [Member] | Spent Nuclear Fuel and Decommissioning Trusts [Member]      
Nuclear Trust Fund Investments      
Gross Unrealized Gains [6] 945.6   766.3
Other-Than-Temporary Impairments [6] 0.0   0.0
Unrealized Gain on Securities 955.0 784.0  
Unrealized Loss on Securities 9.0 $ 18.0  
Mutual Funds Equity [Member]      
Other Temporary Investments      
Cost 22.5   17.8
Gross Unrealized Gains 16.3   16.4
Gross Unrealized Losses 0.0   0.0
Fair Value [5] 38.8   34.2
Cash and Cash Equivalents [Member]      
Nuclear Trust Fund Investments      
Fair Value [7] 17.4   22.5
Contractual Maturities, Fair Value of Debt Securities      
Fair Value [7] 17.4   22.5
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 [7] 17.4   22.5
Contractual Maturities, Fair Value of Debt Securities      
Fair Value [7] 17.4   22.5
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 1,015.6   996.1
Contractual Maturities, Fair Value of Debt Securities      
Fair Value 1,015.6   996.1
US Government Agencies Debt Securities [Member] | Spent Nuclear Fuel and Decommissioning Trusts [Member]      
Nuclear Trust Fund Investments      
Gross Unrealized Gains 44.0   26.7
Other-Than-Temporary Impairments (6.7)   (7.1)
US Government Agencies Debt Securities [Member] | Indiana Michigan Power Co [Member]      
Nuclear Trust Fund Investments      
Fair Value 1,015.6   996.1
Contractual Maturities, Fair Value of Debt Securities      
Fair Value 1,015.6   996.1
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 44.0   26.7
Other-Than-Temporary Impairments (6.7)   (7.1)
Corporate Debt [Member]      
Nuclear Trust Fund Investments      
Fair Value 54.2   52.4
Contractual Maturities, Fair Value of Debt Securities      
Fair Value 54.2   52.4
Corporate Debt [Member] | Spent Nuclear Fuel and Decommissioning Trusts [Member]      
Nuclear Trust Fund Investments      
Gross Unrealized Gains 2.9   1.1
Other-Than-Temporary Impairments (1.8)   (1.9)
Corporate Debt [Member] | Indiana Michigan Power Co [Member]      
Nuclear Trust Fund Investments      
Fair Value 54.2   52.4
Contractual Maturities, Fair Value of Debt Securities      
Fair Value 54.2   52.4
Corporate Debt [Member] | Indiana Michigan Power Co [Member] | Spent Nuclear Fuel and Decommissioning Trusts [Member]      
Nuclear Trust Fund Investments      
Gross Unrealized Gains 2.9   1.1
Other-Than-Temporary Impairments (1.8)   (1.9)
State and Local Jurisdiction [Member]      
Nuclear Trust Fund Investments      
Fair Value 8.7   8.6
Contractual Maturities, Fair Value of Debt Securities      
Fair Value 8.7   8.6
State and Local Jurisdiction [Member] | Spent Nuclear Fuel and Decommissioning Trusts [Member]      
Nuclear Trust Fund Investments      
Gross Unrealized Gains 0.7   0.6
Other-Than-Temporary Impairments (0.2)   (0.2)
State and Local Jurisdiction [Member] | Indiana Michigan Power Co [Member]      
Nuclear Trust Fund Investments      
Fair Value 8.7   8.6
Contractual Maturities, Fair Value of Debt Securities      
Fair Value 8.7   8.6
State and Local Jurisdiction [Member] | Indiana Michigan Power Co [Member] | Spent Nuclear Fuel and Decommissioning Trusts [Member]      
Nuclear Trust Fund Investments      
Gross Unrealized Gains 0.7   0.6
Other-Than-Temporary Impairments (0.2)   $ (0.2)
Fair Value, Inputs, Level 1 [Member]      
Book Values and Fair Values of Long - term Debt      
Long Term Debt, Fair Value $ 828.0    
[1] The fair value amount includes debt related to AEP’s Equity Units issued in March 2019 and has a fair value of $828 million as of March 31, 2019. See “Equity Units” section of Note 13 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 $955 million and $784 million and unrealized losses of $9 million and $18 million as of March 31, 2019 and December 31, 2018, respectively. 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.
[7] Amounts in “Other’’ column primarily represent accrued interest receivables from financial institutions.  Level 1 amounts primarily represent investments in money market funds.