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Benefit Plans 3 (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
American Electric Power System Retirement Savings Plans      
Cost of Company Matching Contributions $ 71.8 $ 74.6 $ 72.9
Benefit Plans Textuals [Abstract]      
Matching Contributions Provided Percentage 100.00%    
Eligible Compensation Contribution by Employee Percentage 1.00%    
Second Matching Contributions Provided Percentage 70.00%    
Second Eligible Compensation Contribution by Employee Percentage 5.00%    
Multiemployer Plan Surcharge   10.00%  
Multiemployer Plans Withdrawal Obligation $ 15.0 $ 19.0  
Noncurrent Regulatory Liabilities 8,540.3 8,422.3  
Noncurrent Regulatory Assets 3,310.4 3,587.6  
UMWA Withdrawal Obligation [Member]      
Benefit Plans Textuals [Abstract]      
Noncurrent Regulatory Liabilities 3.0    
Noncurrent Regulatory Assets   1.0  
AEP Texas Inc. [Member]      
American Electric Power System Retirement Savings Plans      
Cost of Company Matching Contributions $ 5.7 6.0 5.2
Benefit Plans Textuals [Abstract]      
Matching Contributions Provided Percentage 100.00%    
Eligible Compensation Contribution by Employee Percentage 1.00%    
Second Matching Contributions Provided Percentage 70.00%    
Second Eligible Compensation Contribution by Employee Percentage 5.00%    
Noncurrent Regulatory Liabilities $ 1,344.3 1,320.5  
Noncurrent Regulatory Assets 430.0 378.7  
Appalachian Power Co [Member]      
American Electric Power System Retirement Savings Plans      
Cost of Company Matching Contributions $ 7.5 7.4 7.3
Benefit Plans Textuals [Abstract]      
Matching Contributions Provided Percentage 100.00%    
Eligible Compensation Contribution by Employee Percentage 1.00%    
Second Matching Contributions Provided Percentage 70.00%    
Second Eligible Compensation Contribution by Employee Percentage 5.00%    
Noncurrent Regulatory Liabilities $ 1,449.7 1,454.9  
Noncurrent Regulatory Assets 475.8 573.9  
Indiana Michigan Power Co [Member]      
American Electric Power System Retirement Savings Plans      
Cost of Company Matching Contributions $ 10.5 10.7 10.9
Benefit Plans Textuals [Abstract]      
Matching Contributions Provided Percentage 100.00%    
Eligible Compensation Contribution by Employee Percentage 1.00%    
Second Matching Contributions Provided Percentage 70.00%    
Second Eligible Compensation Contribution by Employee Percentage 5.00%    
Noncurrent Regulatory Liabilities $ 1,574.5 1,708.7  
Noncurrent Regulatory Assets 512.5 579.4  
Ohio Power Co [Member]      
American Electric Power System Retirement Savings Plans      
Cost of Company Matching Contributions $ 6.3 6.1 5.6
Benefit Plans Textuals [Abstract]      
Matching Contributions Provided Percentage 100.00%    
Eligible Compensation Contribution by Employee Percentage 1.00%    
Second Matching Contributions Provided Percentage 70.00%    
Second Eligible Compensation Contribution by Employee Percentage 5.00%    
Noncurrent Regulatory Liabilities $ 1,221.2 1,100.2  
Noncurrent Regulatory Assets 387.5 652.8  
Public Service Co Of Oklahoma [Member]      
American Electric Power System Retirement Savings Plans      
Cost of Company Matching Contributions $ 4.5 5.0 4.3
Benefit Plans Textuals [Abstract]      
Matching Contributions Provided Percentage 100.00%    
Eligible Compensation Contribution by Employee Percentage 1.00%    
Second Matching Contributions Provided Percentage 70.00%    
Second Eligible Compensation Contribution by Employee Percentage 5.00%    
Noncurrent Regulatory Liabilities $ 864.7 853.5  
Noncurrent Regulatory Assets 369.0 368.1  
Southwestern Electric Power Co [Member]      
American Electric Power System Retirement Savings Plans      
Cost of Company Matching Contributions $ 5.9 6.0 5.7
Benefit Plans Textuals [Abstract]      
Matching Contributions Provided Percentage 100.00%    
Eligible Compensation Contribution by Employee Percentage 1.00%    
Second Matching Contributions Provided Percentage 70.00%    
Second Eligible Compensation Contribution by Employee Percentage 5.00%    
Noncurrent Regulatory Liabilities $ 923.0 896.4  
Noncurrent Regulatory Assets 230.8 220.6  
Pension Plans [Member]      
Accumulated Benefit Obligation      
Accumulated Benefit Obligation 4,625.6 5,025.2  
Underfunded Benefit Obligation      
Projected Benefit Obligation 4,810.3 5,215.8 5,085.8
Accumulated Benefit Obligation 4,625.6 5,025.2  
Fair Value of Plan Assets 4,695.9 5,174.1 4,827.3
Underfunded Status, Projected Benefit Obligation (114.4)    
Estimated Future Benefit Payments and Contributions      
Expected Contributions and Payments During 2019 98.7    
Components of Net Periodic Benefit Cost      
Service Cost 97.6 96.5 85.8
Interest Cost 187.8 203.1 211.6
Expected Return on Plan Assets (290.3) (284.8) (280.3)
Amortization of Prior Service Cost (Credit) 0.0 1.0 2.3
Amortization of Net Actuarial Loss 85.2 82.8 83.8
Settlements 2.6 0.0 0.0
Net Periodic Benefit Cost (Credit) 82.9 98.6 103.2
Capitalized Portion (41.1) (39.9) (37.8)
Net Periodic Benefit Cost (Credit) Recognized in Expense 41.8 58.7 65.4
Pension Plans [Member] | Pension Payments [Member]      
Estimated Future Benefit Payments and Contributions      
2019 339.8    
2020 344.2    
2021 354.2    
2022 357.3    
2023 364.1    
Years 2024 to 2028, in Total 1,808.2    
Pension Plans [Member] | AEP Texas Inc. [Member]      
Accumulated Benefit Obligation      
Accumulated Benefit Obligation 396.8 425.2  
Underfunded Benefit Obligation      
Projected Benefit Obligation 409.3 441.3 421.7
Accumulated Benefit Obligation 396.8 425.2  
Fair Value of Plan Assets 410.7 455.9 416.6
Estimated Future Benefit Payments and Contributions      
Expected Contributions and Payments During 2019 8.0    
Components of Net Periodic Benefit Cost      
Service Cost 9.2 8.6 7.5
Interest Cost 16.0 17.1 17.8
Expected Return on Plan Assets (25.6) (25.0) (24.5)
Amortization of Prior Service Cost (Credit) 0.0 0.0 0.4
Amortization of Net Actuarial Loss 7.2 7.0 7.1
Net Periodic Benefit Cost (Credit) 6.8 7.7 8.3
Capitalized Portion (4.8) (4.0) (3.6)
Net Periodic Benefit Cost (Credit) Recognized in Expense 2.0 3.7 4.7
Pension Plans [Member] | AEP Texas Inc. [Member] | Pension Payments [Member]      
Estimated Future Benefit Payments and Contributions      
2019 30.8    
2020 34.3    
2021 34.9    
2022 33.5    
2023 34.9    
Years 2024 to 2028, in Total 164.7    
Pension Plans [Member] | Appalachian Power Co [Member]      
Accumulated Benefit Obligation      
Accumulated Benefit Obligation 588.5 648.2  
Underfunded Benefit Obligation      
Projected Benefit Obligation 603.1 665.0 654.0
Accumulated Benefit Obligation 588.5 648.2  
Fair Value of Plan Assets 593.3 651.7 606.4
Underfunded Status, Projected Benefit Obligation (9.8) (13.3)  
Estimated Future Benefit Payments and Contributions      
Expected Contributions and Payments During 2019 7.7    
Components of Net Periodic Benefit Cost      
Service Cost 9.3 9.4 8.1
Interest Cost 23.5 25.7 27.2
Expected Return on Plan Assets (36.6) (35.8) (35.3)
Amortization of Prior Service Cost (Credit) 0.0 0.2 0.1
Amortization of Net Actuarial Loss 10.6 10.4 10.8
Net Periodic Benefit Cost (Credit) 6.8 9.9 10.9
Capitalized Portion (3.8) (4.0) (4.1)
Net Periodic Benefit Cost (Credit) Recognized in Expense 3.0 5.9 6.8
Pension Plans [Member] | Appalachian Power Co [Member] | Pension Payments [Member]      
Estimated Future Benefit Payments and Contributions      
2019 43.4    
2020 42.8    
2021 43.4    
2022 43.6    
2023 44.2    
Years 2024 to 2028, in Total 220.2    
Pension Plans [Member] | Indiana Michigan Power Co [Member]      
Accumulated Benefit Obligation      
Accumulated Benefit Obligation 536.9 592.8  
Underfunded Benefit Obligation      
Projected Benefit Obligation 567.0 624.3 611.6
Accumulated Benefit Obligation 536.9 592.8  
Fair Value of Plan Assets 583.8 636.7 586.1
Estimated Future Benefit Payments and Contributions      
Expected Contributions and Payments During 2019 1.1    
Components of Net Periodic Benefit Cost      
Service Cost 13.6 14.0 12.2
Interest Cost 22.1 24.3 25.3
Expected Return on Plan Assets (35.7) (34.6) (33.6)
Amortization of Prior Service Cost (Credit) 0.0 0.2 0.1
Amortization of Net Actuarial Loss 9.8 9.7 10.0
Net Periodic Benefit Cost (Credit) 9.8 13.6 14.0
Capitalized Portion (5.6) (5.5) (3.3)
Net Periodic Benefit Cost (Credit) Recognized in Expense 4.2 8.1 10.7
Pension Plans [Member] | Indiana Michigan Power Co [Member] | Pension Payments [Member]      
Estimated Future Benefit Payments and Contributions      
2019 36.2    
2020 36.4    
2021 37.5    
2022 38.9    
2023 40.3    
Years 2024 to 2028, in Total 210.6    
Pension Plans [Member] | Ohio Power Co [Member]      
Accumulated Benefit Obligation      
Accumulated Benefit Obligation 438.5 483.5  
Underfunded Benefit Obligation      
Projected Benefit Obligation 453.9 501.1 492.9
Accumulated Benefit Obligation 438.5 483.5  
Fair Value of Plan Assets 466.1 509.1 473.8
Estimated Future Benefit Payments and Contributions      
Expected Contributions and Payments During 2019 0.5    
Components of Net Periodic Benefit Cost      
Service Cost 7.7 7.5 6.5
Interest Cost 17.7 19.4 20.6
Expected Return on Plan Assets (28.8) (27.9) (27.6)
Amortization of Prior Service Cost (Credit) 0.0 0.1 0.1
Amortization of Net Actuarial Loss 8.0 7.8 8.1
Net Periodic Benefit Cost (Credit) 4.6 6.9 7.7
Capitalized Portion (3.6) (3.3) (3.4)
Net Periodic Benefit Cost (Credit) Recognized in Expense 1.0 3.6 4.3
Pension Plans [Member] | Ohio Power Co [Member] | Pension Payments [Member]      
Estimated Future Benefit Payments and Contributions      
2019 34.3    
2020 34.5    
2021 34.0    
2022 33.9    
2023 34.7    
Years 2024 to 2028, in Total 163.3    
Pension Plans [Member] | Public Service Co Of Oklahoma [Member]      
Accumulated Benefit Obligation      
Accumulated Benefit Obligation 240.2 259.6  
Underfunded Benefit Obligation      
Projected Benefit Obligation 253.8 276.6 266.7
Accumulated Benefit Obligation 240.2 259.6  
Fair Value of Plan Assets 261.2 287.8 266.0
Estimated Future Benefit Payments and Contributions      
Expected Contributions and Payments During 2019 0.2    
Components of Net Periodic Benefit Cost      
Service Cost 7.0 6.4 6.2
Interest Cost 9.9 10.7 11.2
Expected Return on Plan Assets (16.1) (15.6) (15.5)
Amortization of Prior Service Cost (Credit) 0.0 0.0 0.3
Amortization of Net Actuarial Loss 4.4 4.3 4.4
Net Periodic Benefit Cost (Credit) 5.2 5.8 6.6
Capitalized Portion (2.6) (2.1) (2.4)
Net Periodic Benefit Cost (Credit) Recognized in Expense 2.6 3.7 4.2
Pension Plans [Member] | Public Service Co Of Oklahoma [Member] | Pension Payments [Member]      
Estimated Future Benefit Payments and Contributions      
2019 19.0    
2020 19.5    
2021 21.2    
2022 20.4    
2023 23.1    
Years 2024 to 2028, in Total 102.5    
Pension Plans [Member] | Southwestern Electric Power Co [Member]      
Accumulated Benefit Obligation      
Accumulated Benefit Obligation 272.8 291.6  
Underfunded Benefit Obligation      
Projected Benefit Obligation 291.4 314.6 296.6
Accumulated Benefit Obligation 272.8 291.6  
Fair Value of Plan Assets 281.0 311.7 287.3
Underfunded Status, Projected Benefit Obligation (10.4) (2.9)  
Estimated Future Benefit Payments and Contributions      
Expected Contributions and Payments During 2019 7.9    
Components of Net Periodic Benefit Cost      
Service Cost 9.3 8.7 8.1
Interest Cost 11.3 12.3 12.4
Expected Return on Plan Assets (17.3) (17.0) (16.4)
Amortization of Prior Service Cost (Credit) 0.0 0.1 0.3
Amortization of Net Actuarial Loss 5.1 4.9 4.8
Settlements 0.4 0.0 0.0
Net Periodic Benefit Cost (Credit) 8.8 9.0 9.2
Capitalized Portion (3.1) (2.7) (2.7)
Net Periodic Benefit Cost (Credit) Recognized in Expense 5.7 6.3 6.5
Pension Plans [Member] | Southwestern Electric Power Co [Member] | Pension Payments [Member]      
Estimated Future Benefit Payments and Contributions      
2019 21.4    
2020 21.8    
2021 22.9    
2022 23.8    
2023 24.0    
Years 2024 to 2028, in Total 120.5    
Qualified Pension Plans [Member]      
Accumulated Benefit Obligation      
Accumulated Benefit Obligation 4,560.7 4,951.3  
Underfunded Benefit Obligation      
Accumulated Benefit Obligation 4,560.7 4,951.3  
Qualified Pension Plans [Member] | AEP Texas Inc. [Member]      
Accumulated Benefit Obligation      
Accumulated Benefit Obligation 393.2 421.4  
Underfunded Benefit Obligation      
Accumulated Benefit Obligation 393.2 421.4  
Qualified Pension Plans [Member] | Appalachian Power Co [Member]      
Accumulated Benefit Obligation      
Accumulated Benefit Obligation 588.3 648.0  
Underfunded Benefit Obligation      
Accumulated Benefit Obligation 588.3 648.0  
Qualified Pension Plans [Member] | Indiana Michigan Power Co [Member]      
Accumulated Benefit Obligation      
Accumulated Benefit Obligation 536.3 592.4  
Underfunded Benefit Obligation      
Accumulated Benefit Obligation 536.3 592.4  
Qualified Pension Plans [Member] | Ohio Power Co [Member]      
Accumulated Benefit Obligation      
Accumulated Benefit Obligation 438.3 483.4  
Underfunded Benefit Obligation      
Accumulated Benefit Obligation 438.3 483.4  
Qualified Pension Plans [Member] | Public Service Co Of Oklahoma [Member]      
Accumulated Benefit Obligation      
Accumulated Benefit Obligation 238.0 256.9  
Underfunded Benefit Obligation      
Accumulated Benefit Obligation 238.0 256.9  
Qualified Pension Plans [Member] | Southwestern Electric Power Co [Member]      
Accumulated Benefit Obligation      
Accumulated Benefit Obligation 271.6 289.4  
Underfunded Benefit Obligation      
Accumulated Benefit Obligation 271.6 289.4  
Nonqualified Pension Plans [Member]      
Accumulated Benefit Obligation      
Accumulated Benefit Obligation 64.9 73.9  
Underfunded Benefit Obligation      
Projected Benefit Obligation   78.0  
Accumulated Benefit Obligation 64.9 73.9  
Fair Value of Plan Assets 0.0 0.0  
Underfunded Status, Projected Benefit Obligation   (78.0)  
Underfunded Status, Accumulated Benefit Obligation (64.9) (73.9)  
Nonqualified Pension Plans [Member] | AEP Texas Inc. [Member]      
Accumulated Benefit Obligation      
Accumulated Benefit Obligation 3.6 3.8  
Underfunded Benefit Obligation      
Projected Benefit Obligation 3.8 4.0  
Accumulated Benefit Obligation 3.6 3.8  
Fair Value of Plan Assets 0.0 0.0  
Underfunded Status, Projected Benefit Obligation (3.8) (4.0)  
Underfunded Status, Accumulated Benefit Obligation (3.6) (3.8)  
Nonqualified Pension Plans [Member] | Appalachian Power Co [Member]      
Accumulated Benefit Obligation      
Accumulated Benefit Obligation 0.2 0.2  
Underfunded Benefit Obligation      
Accumulated Benefit Obligation 0.2 0.2  
Fair Value of Plan Assets 0.0 0.0  
Underfunded Status, Accumulated Benefit Obligation (0.2) (0.2)  
Nonqualified Pension Plans [Member] | Indiana Michigan Power Co [Member]      
Accumulated Benefit Obligation      
Accumulated Benefit Obligation 0.6 0.4  
Underfunded Benefit Obligation      
Projected Benefit Obligation 1.2 1.0  
Accumulated Benefit Obligation 0.6 0.4  
Fair Value of Plan Assets 0.0 0.0  
Underfunded Status, Projected Benefit Obligation (1.2) (1.0)  
Underfunded Status, Accumulated Benefit Obligation (0.6) (0.4)  
Nonqualified Pension Plans [Member] | Ohio Power Co [Member]      
Accumulated Benefit Obligation      
Accumulated Benefit Obligation 0.2 0.1  
Underfunded Benefit Obligation      
Projected Benefit Obligation 0.4 0.4  
Accumulated Benefit Obligation 0.2 0.1  
Fair Value of Plan Assets 0.0 0.0  
Underfunded Status, Projected Benefit Obligation (0.4) (0.4)  
Underfunded Status, Accumulated Benefit Obligation (0.2) (0.1)  
Nonqualified Pension Plans [Member] | Public Service Co Of Oklahoma [Member]      
Accumulated Benefit Obligation      
Accumulated Benefit Obligation 2.2 2.7  
Underfunded Benefit Obligation      
Projected Benefit Obligation 2.3 2.7  
Accumulated Benefit Obligation 2.2 2.7  
Fair Value of Plan Assets 0.0 0.0  
Underfunded Status, Projected Benefit Obligation (2.3) (2.7)  
Underfunded Status, Accumulated Benefit Obligation (2.2) (2.7)  
Nonqualified Pension Plans [Member] | Southwestern Electric Power Co [Member]      
Accumulated Benefit Obligation      
Accumulated Benefit Obligation 1.2 2.2  
Underfunded Benefit Obligation      
Accumulated Benefit Obligation 1.2 2.2  
Fair Value of Plan Assets 0.0 0.0  
Underfunded Status, Accumulated Benefit Obligation (1.2) (2.2)  
Other Postretirement Benefit Plans [Member]      
Underfunded Benefit Obligation      
Projected Benefit Obligation 1,194.5 1,332.0 1,447.4
Fair Value of Plan Assets 1,534.2 1,732.5 1,545.9
Estimated Future Benefit Payments and Contributions      
Expected Contributions and Payments During 2019 4.5    
Components of Net Periodic Benefit Cost      
Service Cost 11.6 11.2 10.2
Interest Cost 47.4 59.3 60.9
Expected Return on Plan Assets (102.2) (101.3) (107.0)
Amortization of Prior Service Cost (Credit) (69.1) (69.1) (69.0)
Amortization of Net Actuarial Loss 10.5 36.7 31.4
Settlements 0.0 0.0 0.0
Net Periodic Benefit Cost (Credit) (101.8) (63.2) (73.5)
Capitalized Portion (4.9) 25.6 26.9
Net Periodic Benefit Cost (Credit) Recognized in Expense (106.7) (37.6) (46.6)
Other Postretirement Benefit Plans [Member] | Benefit Payments [Member]      
Estimated Future Benefit Payments and Contributions      
2019 122.0    
2020 126.5    
2021 127.1    
2022 127.2    
2023 126.3    
Years 2024 to 2028, in Total 618.8    
Other Postretirement Benefit Plans [Member] | Medicare Subsidy Receipts [Member]      
Estimated Future Benefit Payments and Contributions      
2019 0.2    
2020 0.2    
2021 0.3    
2022 0.3    
2023 0.3    
Years 2024 to 2028, in Total 1.5    
Other Postretirement Benefit Plans [Member] | AEP Texas Inc. [Member]      
Underfunded Benefit Obligation      
Projected Benefit Obligation 95.9 107.1 120.4
Fair Value of Plan Assets 129.9 147.3 134.1
Estimated Future Benefit Payments and Contributions      
Expected Contributions and Payments During 2019 0.1    
Components of Net Periodic Benefit Cost      
Service Cost 0.9 0.9 0.7
Interest Cost 3.8 4.9 5.1
Expected Return on Plan Assets (8.6) (8.8) (9.3)
Amortization of Prior Service Cost (Credit) (5.9) (5.8) (6.0)
Amortization of Net Actuarial Loss 0.8 3.2 2.8
Net Periodic Benefit Cost (Credit) (9.0) (5.6) (6.7)
Capitalized Portion (0.5) 2.9 3.4
Net Periodic Benefit Cost (Credit) Recognized in Expense (9.5) (2.7) (3.3)
Other Postretirement Benefit Plans [Member] | AEP Texas Inc. [Member] | Benefit Payments [Member]      
Estimated Future Benefit Payments and Contributions      
2019 10.0    
2020 10.5    
2021 10.7    
2022 10.9    
2023 10.9    
Years 2024 to 2028, in Total 53.6    
Other Postretirement Benefit Plans [Member] | AEP Texas Inc. [Member] | Medicare Subsidy Receipts [Member]      
Estimated Future Benefit Payments and Contributions      
2019 0.0    
2020 0.0    
2021 0.0    
2022 0.0    
2023 0.0    
Years 2024 to 2028, in Total 0.0    
Other Postretirement Benefit Plans [Member] | Appalachian Power Co [Member]      
Underfunded Benefit Obligation      
Projected Benefit Obligation 205.5 236.5 255.6
Fair Value of Plan Assets 238.4 273.4 246.9
Estimated Future Benefit Payments and Contributions      
Expected Contributions and Payments During 2019 2.1    
Components of Net Periodic Benefit Cost      
Service Cost 1.1 1.1 1.0
Interest Cost 8.2 10.6 10.8
Expected Return on Plan Assets (16.0) (16.5) (17.3)
Amortization of Prior Service Cost (Credit) (10.0) (10.1) (10.1)
Amortization of Net Actuarial Loss 1.9 6.3 5.4
Net Periodic Benefit Cost (Credit) (14.8) (8.6) (10.2)
Capitalized Portion (0.5) 3.5 3.9
Net Periodic Benefit Cost (Credit) Recognized in Expense (15.3) (5.1) (6.3)
Other Postretirement Benefit Plans [Member] | Appalachian Power Co [Member] | Benefit Payments [Member]      
Estimated Future Benefit Payments and Contributions      
2019 22.0    
2020 22.5    
2021 22.2    
2022 22.1    
2023 21.6    
Years 2024 to 2028, in Total 103.4    
Other Postretirement Benefit Plans [Member] | Appalachian Power Co [Member] | Medicare Subsidy Receipts [Member]      
Estimated Future Benefit Payments and Contributions      
2019 0.2    
2020 0.2    
2021 0.2    
2022 0.2    
2023 0.2    
Years 2024 to 2028, in Total 0.7    
Other Postretirement Benefit Plans [Member] | Indiana Michigan Power Co [Member]      
Underfunded Benefit Obligation      
Projected Benefit Obligation 138.3 153.5 167.6
Fair Value of Plan Assets 187.3 211.1 186.6
Estimated Future Benefit Payments and Contributions      
Expected Contributions and Payments During 2019 0.0    
Components of Net Periodic Benefit Cost      
Service Cost 1.6 1.6 1.5
Interest Cost 5.4 6.9 7.0
Expected Return on Plan Assets (12.3) (12.2) (12.9)
Amortization of Prior Service Cost (Credit) (9.5) (9.4) (9.4)
Amortization of Net Actuarial Loss 1.2 4.4 3.7
Net Periodic Benefit Cost (Credit) (13.6) (8.7) (10.1)
Capitalized Portion (0.7) 3.5 2.4
Net Periodic Benefit Cost (Credit) Recognized in Expense (14.3) (5.2) (7.7)
Other Postretirement Benefit Plans [Member] | Indiana Michigan Power Co [Member] | Benefit Payments [Member]      
Estimated Future Benefit Payments and Contributions      
2019 14.8    
2020 15.4    
2021 15.7    
2022 15.7    
2023 15.6    
Years 2024 to 2028, in Total 75.6    
Other Postretirement Benefit Plans [Member] | Indiana Michigan Power Co [Member] | Medicare Subsidy Receipts [Member]      
Estimated Future Benefit Payments and Contributions      
2019 0.0    
2020 0.0    
2021 0.0    
2022 0.0    
2023 0.0    
Years 2024 to 2028, in Total 0.0    
Other Postretirement Benefit Plans [Member] | Ohio Power Co [Member]      
Underfunded Benefit Obligation      
Projected Benefit Obligation 129.5 144.3 164.0
Fair Value of Plan Assets 175.4 198.5 182.6
Estimated Future Benefit Payments and Contributions      
Expected Contributions and Payments During 2019 0.0    
Components of Net Periodic Benefit Cost      
Service Cost 0.9 0.9 0.8
Interest Cost 5.1 6.7 7.0
Expected Return on Plan Assets (11.7) (11.9) (13.0)
Amortization of Prior Service Cost (Credit) (6.9) (6.9) (6.9)
Amortization of Net Actuarial Loss 1.1 4.3 3.8
Net Periodic Benefit Cost (Credit) (11.5) (6.9) (8.3)
Capitalized Portion (0.4) 3.3 3.7
Net Periodic Benefit Cost (Credit) Recognized in Expense (11.9) (3.6) (4.6)
Other Postretirement Benefit Plans [Member] | Ohio Power Co [Member] | Benefit Payments [Member]      
Estimated Future Benefit Payments and Contributions      
2019 14.5    
2020 14.8    
2021 14.8    
2022 14.7    
2023 14.5    
Years 2024 to 2028, in Total 69.1    
Other Postretirement Benefit Plans [Member] | Ohio Power Co [Member] | Medicare Subsidy Receipts [Member]      
Estimated Future Benefit Payments and Contributions      
2019 0.0    
2020 0.0    
2021 0.0    
2022 0.0    
2023 0.0    
Years 2024 to 2028, in Total 0.0    
Other Postretirement Benefit Plans [Member] | Public Service Co Of Oklahoma [Member]      
Underfunded Benefit Obligation      
Projected Benefit Obligation 62.3 69.4 77.6
Fair Value of Plan Assets 84.3 95.5 86.4
Estimated Future Benefit Payments and Contributions      
Expected Contributions and Payments During 2019 0.0    
Components of Net Periodic Benefit Cost      
Service Cost 0.7 0.7 0.6
Interest Cost 2.5 3.2 3.3
Expected Return on Plan Assets (5.6) (5.6) (6.1)
Amortization of Prior Service Cost (Credit) (4.3) (4.3) (4.3)
Amortization of Net Actuarial Loss 0.5 2.0 1.8
Net Periodic Benefit Cost (Credit) (6.2) (4.0) (4.7)
Capitalized Portion (0.3) 1.4 1.7
Net Periodic Benefit Cost (Credit) Recognized in Expense (6.5) (2.6) (3.0)
Other Postretirement Benefit Plans [Member] | Public Service Co Of Oklahoma [Member] | Benefit Payments [Member]      
Estimated Future Benefit Payments and Contributions      
2019 6.3    
2020 6.8    
2021 6.8    
2022 7.0    
2023 7.1    
Years 2024 to 2028, in Total 34.9    
Other Postretirement Benefit Plans [Member] | Public Service Co Of Oklahoma [Member] | Medicare Subsidy Receipts [Member]      
Estimated Future Benefit Payments and Contributions      
2019 0.0    
2020 0.0    
2021 0.0    
2022 0.0    
2023 0.0    
Years 2024 to 2028, in Total 0.0    
Other Postretirement Benefit Plans [Member] | Southwestern Electric Power Co [Member]      
Underfunded Benefit Obligation      
Projected Benefit Obligation 72.7 80.3 86.9
Fair Value of Plan Assets 98.5 110.4 96.8
Estimated Future Benefit Payments and Contributions      
Expected Contributions and Payments During 2019 0.0    
Components of Net Periodic Benefit Cost      
Service Cost 0.9 0.9 0.8
Interest Cost 2.8 3.6 3.6
Expected Return on Plan Assets (6.4) (6.3) (6.8)
Amortization of Prior Service Cost (Credit) (5.2) (5.2) (5.0)
Amortization of Net Actuarial Loss 0.6 2.3 1.9
Settlements 0.0 0.0 0.0
Net Periodic Benefit Cost (Credit) (7.3) (4.7) (5.5)
Capitalized Portion (0.3) 1.4 1.6
Net Periodic Benefit Cost (Credit) Recognized in Expense (7.6) $ (3.3) $ (3.9)
Other Postretirement Benefit Plans [Member] | Southwestern Electric Power Co [Member] | Benefit Payments [Member]      
Estimated Future Benefit Payments and Contributions      
2019 7.1    
2020 7.5    
2021 7.8    
2022 8.0    
2023 8.1    
Years 2024 to 2028, in Total 41.5    
Other Postretirement Benefit Plans [Member] | Southwestern Electric Power Co [Member] | Medicare Subsidy Receipts [Member]      
Estimated Future Benefit Payments and Contributions      
2019 0.0    
2020 0.0    
2021 0.0    
2022 0.0    
2023 0.0    
Years 2024 to 2028, in Total $ 0.0