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Benefit Plans 1 (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Benefit Amounts Recognized on the Balance Sheets      
Deferred Charges and Other Noncurrent Assets - Prepaid Benefit Costs $ 2,553.5 $ 2,085.1  
Deferred Credits and Other Noncurrent Liabilities - Accrued Long-term Benefit Liability (830.9) (774.6)  
AEP Texas Inc. [Member]      
Benefit Amounts Recognized on the Balance Sheets      
Deferred Charges and Other Noncurrent Assets - Prepaid Benefit Costs 114.8 73.3  
Deferred Credits and Other Noncurrent Liabilities - Accrued Long-term Benefit Liability (63.4) (56.3)  
Appalachian Power Co [Member]      
Benefit Amounts Recognized on the Balance Sheets      
Deferred Charges and Other Noncurrent Assets - Prepaid Benefit Costs 190.0 133.3  
Deferred Credits and Other Noncurrent Liabilities - Accrued Long-term Benefit Liability (74.7) (64.5)  
Indiana Michigan Power Co [Member]      
Benefit Amounts Recognized on the Balance Sheets      
Deferred Charges and Other Noncurrent Assets - Prepaid Benefit Costs 179.9 121.5  
Deferred Credits and Other Noncurrent Liabilities - Accrued Long-term Benefit Liability (88.5) (120.4)  
Ohio Power Co [Member]      
Benefit Amounts Recognized on the Balance Sheets      
Deferred Charges and Other Noncurrent Assets - Prepaid Benefit Costs 374.2 295.5  
Deferred Credits and Other Noncurrent Liabilities - Accrued Long-term Benefit Liability (46.2) (111.7)  
Public Service Co Of Oklahoma [Member]      
Benefit Amounts Recognized on the Balance Sheets      
Deferred Charges and Other Noncurrent Assets - Prepaid Benefit Costs 8.7 10.0  
Deferred Credits and Other Noncurrent Liabilities - Accrued Long-term Benefit Liability (22.5) (24.8)  
Southwestern Electric Power Co [Member]      
Benefit Amounts Recognized on the Balance Sheets      
Deferred Charges and Other Noncurrent Assets - Prepaid Benefit Costs 109.9 99.9  
Deferred Credits and Other Noncurrent Liabilities - Accrued Long-term Benefit Liability (19.9) (9.7)  
Pension Plans [Member]      
Underfunded Status, Accumulated Benefit Obligation   (88.5)  
Defined Benefit Plan, Accumulated Benefit Obligation $ 5,025.2 $ 4,915.8  
Actuarial Assumptions for Benefit Obligations      
Discount Rate 3.65% 4.05%  
Rate of Compensation Increase [1] 4.80% 4.75%  
Actuarial Assumptions for Net Periodic Benefit Costs      
Discount Rate 4.05% 4.30% 4.00%
Expected Return on Plan Assets 6.00% 6.00% 6.00%
Rate of Compensation Increase [2] 4.80% 4.75% 4.80%
Change in Benefit Obligation      
Benefit Obligation as of January 1 $ 5,085.8 $ 4,992.9  
Service Cost 96.5 85.8 $ 93.5
Interest Cost 203.1 211.6 205.3
Actuarial (Gain) Loss 182.4 142.7  
Benefit Payments (352.0) (347.2)  
Participant Contributions 0.0 0.0  
Medicare Subsidy 0.0 0.0  
Benefit Obligation as of December 31 5,215.8 5,085.8 4,992.9
Change in Fair Value of Plan Assets      
Fair Value of Plan Assets as of January 1 4,827.3 4,767.6  
Actual Gain (Loss) on Plan Assets 600.0 315.5  
Company Contributions 98.8 91.4  
Participant Contributions 0.0 0.0  
Benefit Payments (352.0) (347.2)  
Fair Value of Plan Assets as of December 31 5,174.1 4,827.3 $ 4,767.6
Funded (Underfunded) Status as of December 31 (41.7) (258.5)  
Benefit Amounts Recognized on the Balance Sheets      
Deferred Charges and Other Noncurrent Assets - Prepaid Benefit Costs 36.3 0.0  
Other Current Liabilities - Accrued Short-term Benefit Liability (6.2) (5.9)  
Employee Benefits and Pension Obligations - Accrued Long-term Benefit Liability (71.8) (252.6)  
Funded (Underfunded) Status (41.7) (258.5)  
Components      
Net Actuarial Loss 1,354.2 1,569.8  
Prior Service Cost (Credit) 0.0 1.0  
Components of Change in Amounts Included in AOCI and Regulatory Assets      
Actuarial (Gain) Loss During the Year (132.8) 107.5  
Amortization of Actuarial Gain (Loss) (82.8) (83.8)  
Amortization of Prior Service Credit (Cost) (1.0) (2.3)  
Change for the Year $ (216.6) $ 21.4  
Benefit Plans (Textuals) [Abstract]      
Rate of Compensation Increase [1] 4.80% 4.75%  
Pension Plans [Member] | Income Tax Expense [Member]      
Recorded as      
Total of Amounts Included in AOCI, Income Tax Expense and Regulatory Assets [3] $ 11.6 $ 0.0  
Pension Plans [Member] | Regulatory Assets [Member]      
Recorded as      
Total of Amounts Included in AOCI, Income Tax Expense and Regulatory Assets 1,271.3 1,415.6  
Pension Plans [Member] | Deferred Income Taxes [Member]      
Recorded as      
Total of Amounts Included in AOCI, Income Tax Expense and Regulatory Assets 17.4 54.4  
Pension Plans [Member] | Net of Tax AOCI [Member]      
Recorded as      
Total of Amounts Included in AOCI, Income Tax Expense and Regulatory Assets 53.9 100.8  
Pension Plans [Member] | AEP Texas Inc. [Member]      
Defined Benefit Plan, Accumulated Benefit Obligation $ 425.2 $ 408.5  
Actuarial Assumptions for Benefit Obligations      
Discount Rate 3.65% 4.05%  
Rate of Compensation Increase [1] 4.90% 4.85%  
Actuarial Assumptions for Net Periodic Benefit Costs      
Discount Rate 4.05% 4.30% 4.00%
Expected Return on Plan Assets 6.00% 6.00% 6.00%
Rate of Compensation Increase [2] 4.90% 4.85% 4.50%
Change in Benefit Obligation      
Benefit Obligation as of January 1 $ 421.7 $ 420.3  
Transfer of CSW Energy, Inc. Benefit Obligation 0.0 (2.8)  
Service Cost 8.6 7.5 $ 7.6
Interest Cost 17.1 17.8 17.2
Actuarial (Gain) Loss 25.6 11.1  
Benefit Payments (31.7) (32.2)  
Participant Contributions 0.0 0.0  
Medicare Subsidy 0.0 0.0  
Benefit Obligation as of December 31 441.3 421.7 420.3
Change in Fair Value of Plan Assets      
Fair Value of Plan Assets as of January 1 416.6 415.4  
Transfer of CSW Energy, Inc. Plan Assets 0.0 (2.5)  
Actual Gain (Loss) on Plan Assets 61.8 27.4  
Company Contributions 9.2 8.5  
Participant Contributions 0.0 0.0  
Benefit Payments (31.7) (32.2)  
Fair Value of Plan Assets as of December 31 455.9 416.6 $ 415.4
Funded (Underfunded) Status as of December 31 14.6 (5.1)  
Benefit Amounts Recognized on the Balance Sheets      
Deferred Charges and Other Noncurrent Assets - Prepaid Benefit Costs 18.6 0.0  
Other Current Liabilities - Accrued Short-term Benefit Liability (0.4) (0.4)  
Deferred Credits and Other Noncurrent Liabilities - Accrued Long-term Benefit Liability (3.6) (4.7)  
Funded (Underfunded) Status 14.6 (5.1)  
Components      
Net Actuarial Loss 175.2 193.3  
Prior Service Cost (Credit) 0.0 0.0  
Components of Change in Amounts Included in AOCI and Regulatory Assets      
Actuarial (Gain) Loss During the Year (11.1) 7.1  
Amortization of Actuarial Gain (Loss) (7.0) (7.1)  
Amortization of Prior Service Credit (Cost) 0.0 (0.4)  
Change for the Year $ (18.1) $ (0.4)  
Benefit Plans (Textuals) [Abstract]      
Rate of Compensation Increase [1] 4.90% 4.85%  
Pension Plans [Member] | AEP Texas Inc. [Member] | Income Tax Expense [Member]      
Recorded as      
Total of Amounts Included in AOCI, Income Tax Expense and Regulatory Assets [3] $ 2.0 $ 0.0  
Pension Plans [Member] | AEP Texas Inc. [Member] | Regulatory Assets [Member]      
Recorded as      
Total of Amounts Included in AOCI, Income Tax Expense and Regulatory Assets 161.4 178.5  
Pension Plans [Member] | AEP Texas Inc. [Member] | Deferred Income Taxes [Member]      
Recorded as      
Total of Amounts Included in AOCI, Income Tax Expense and Regulatory Assets 2.9 5.2  
Pension Plans [Member] | AEP Texas Inc. [Member] | Net of Tax AOCI [Member]      
Recorded as      
Total of Amounts Included in AOCI, Income Tax Expense and Regulatory Assets 8.9 9.6  
Pension Plans [Member] | Appalachian Power Co [Member]      
Underfunded Status, Accumulated Benefit Obligation   (34.9)  
Defined Benefit Plan, Accumulated Benefit Obligation $ 648.2 $ 641.3  
Actuarial Assumptions for Benefit Obligations      
Discount Rate 3.65% 4.05%  
Rate of Compensation Increase [1] 4.60% 4.55%  
Actuarial Assumptions for Net Periodic Benefit Costs      
Discount Rate 4.05% 4.30% 4.00%
Expected Return on Plan Assets 6.00% 6.00% 6.00%
Rate of Compensation Increase [2] 4.60% 4.55% 4.45%
Change in Benefit Obligation      
Benefit Obligation as of January 1 $ 654.0 $ 653.4  
Service Cost 9.4 8.1 $ 8.7
Interest Cost 25.7 27.2 26.7
Actuarial (Gain) Loss 15.7 9.2  
Benefit Payments (39.8) (43.9)  
Participant Contributions 0.0 0.0  
Medicare Subsidy 0.0 0.0  
Benefit Obligation as of December 31 665.0 654.0 653.4
Change in Fair Value of Plan Assets      
Fair Value of Plan Assets as of January 1 606.4 603.2  
Actual Gain (Loss) on Plan Assets 74.9 38.3  
Company Contributions 10.2 8.8  
Participant Contributions 0.0 0.0  
Benefit Payments (39.8) (43.9)  
Fair Value of Plan Assets as of December 31 651.7 606.4 $ 603.2
Funded (Underfunded) Status as of December 31 (13.3) (47.6)  
Benefit Amounts Recognized on the Balance Sheets      
Deferred Charges and Other Noncurrent Assets - Prepaid Benefit Costs 0.0 0.0  
Other Current Liabilities - Accrued Short-term Benefit Liability 0.0 0.0  
Employee Benefits and Pension Obligations - Accrued Long-term Benefit Liability (13.3) (47.6)  
Funded (Underfunded) Status (13.3) (47.6)  
Components      
Net Actuarial Loss 182.5 216.2  
Prior Service Cost (Credit) 0.0 0.2  
Components of Change in Amounts Included in AOCI and Regulatory Assets      
Actuarial (Gain) Loss During the Year (23.3) 6.2  
Amortization of Actuarial Gain (Loss) (10.4) (10.8)  
Amortization of Prior Service Credit (Cost) (0.2) (0.1)  
Change for the Year $ (33.9) $ (4.7)  
Benefit Plans (Textuals) [Abstract]      
Rate of Compensation Increase [1] 4.60% 4.55%  
Pension Plans [Member] | Appalachian Power Co [Member] | Income Tax Expense [Member]      
Recorded as      
Total of Amounts Included in AOCI, Income Tax Expense and Regulatory Assets [3] $ 0.4 $ 0.0  
Pension Plans [Member] | Appalachian Power Co [Member] | Regulatory Assets [Member]      
Recorded as      
Total of Amounts Included in AOCI, Income Tax Expense and Regulatory Assets 179.9 213.7  
Pension Plans [Member] | Appalachian Power Co [Member] | Deferred Income Taxes [Member]      
Recorded as      
Total of Amounts Included in AOCI, Income Tax Expense and Regulatory Assets 0.5 1.0  
Pension Plans [Member] | Appalachian Power Co [Member] | Net of Tax AOCI [Member]      
Recorded as      
Total of Amounts Included in AOCI, Income Tax Expense and Regulatory Assets 1.7 1.7  
Pension Plans [Member] | Indiana Michigan Power Co [Member]      
Underfunded Status, Accumulated Benefit Obligation   (2.7)  
Defined Benefit Plan, Accumulated Benefit Obligation $ 592.8 $ 588.8  
Actuarial Assumptions for Benefit Obligations      
Discount Rate 3.65% 4.05%  
Rate of Compensation Increase [1] 4.85% 4.80%  
Actuarial Assumptions for Net Periodic Benefit Costs      
Discount Rate 4.05% 4.30% 4.00%
Expected Return on Plan Assets 6.00% 6.00% 6.00%
Rate of Compensation Increase [2] 4.85% 4.80% 4.80%
Change in Benefit Obligation      
Benefit Obligation as of January 1 $ 611.6 $ 591.5  
Service Cost 14.0 12.2 $ 12.9
Interest Cost 24.3 25.3 24.5
Actuarial (Gain) Loss 10.8 20.1  
Benefit Payments (36.4) (37.5)  
Participant Contributions 0.0 0.0  
Medicare Subsidy 0.0 0.0  
Benefit Obligation as of December 31 624.3 611.6 591.5
Change in Fair Value of Plan Assets      
Fair Value of Plan Assets as of January 1 586.1 570.0  
Actual Gain (Loss) on Plan Assets 74.0 40.6  
Company Contributions 13.0 13.0  
Participant Contributions 0.0 0.0  
Benefit Payments (36.4) (37.5)  
Fair Value of Plan Assets as of December 31 636.7 586.1 $ 570.0
Funded (Underfunded) Status as of December 31 12.4 (25.5)  
Benefit Amounts Recognized on the Balance Sheets      
Deferred Charges and Other Noncurrent Assets - Prepaid Benefit Costs 13.4 0.0  
Deferred Credits and Other Noncurrent Liabilities - Accrued Long-term Benefit Liability (1.0) (25.5)  
Funded (Underfunded) Status 12.4 (25.5)  
Components      
Net Actuarial Loss 94.9 133.2  
Prior Service Cost (Credit) 0.0 0.2  
Components of Change in Amounts Included in AOCI and Regulatory Assets      
Actuarial (Gain) Loss During the Year (28.6) 13.2  
Amortization of Actuarial Gain (Loss) (9.7) (10.0)  
Amortization of Prior Service Credit (Cost) (0.2) (0.1)  
Change for the Year $ (38.5) $ 3.1  
Benefit Plans (Textuals) [Abstract]      
Rate of Compensation Increase [1] 4.85% 4.80%  
Pension Plans [Member] | Indiana Michigan Power Co [Member] | Income Tax Expense [Member]      
Recorded as      
Total of Amounts Included in AOCI, Income Tax Expense and Regulatory Assets [3] $ 0.4 $ 0.0  
Pension Plans [Member] | Indiana Michigan Power Co [Member] | Regulatory Assets [Member]      
Recorded as      
Total of Amounts Included in AOCI, Income Tax Expense and Regulatory Assets 91.8 128.2  
Pension Plans [Member] | Indiana Michigan Power Co [Member] | Deferred Income Taxes [Member]      
Recorded as      
Total of Amounts Included in AOCI, Income Tax Expense and Regulatory Assets 0.7 1.8  
Pension Plans [Member] | Indiana Michigan Power Co [Member] | Net of Tax AOCI [Member]      
Recorded as      
Total of Amounts Included in AOCI, Income Tax Expense and Regulatory Assets 2.0 3.4  
Pension Plans [Member] | Ohio Power Co [Member]      
Underfunded Status, Accumulated Benefit Obligation   (4.2)  
Defined Benefit Plan, Accumulated Benefit Obligation $ 483.5 $ 478.0  
Actuarial Assumptions for Benefit Obligations      
Discount Rate 3.65% 4.05%  
Rate of Compensation Increase [1] 4.95% 4.85%  
Actuarial Assumptions for Net Periodic Benefit Costs      
Discount Rate 4.05% 4.30% 4.00%
Expected Return on Plan Assets 6.00% 6.00% 6.00%
Rate of Compensation Increase [2] 4.95% 4.85% 4.80%
Change in Benefit Obligation      
Benefit Obligation as of January 1 $ 492.9 $ 497.5  
Service Cost 7.5 6.5 $ 6.7
Interest Cost 19.4 20.6 20.3
Actuarial (Gain) Loss 13.1 4.7  
Benefit Payments (31.8) (36.4)  
Participant Contributions 0.0 0.0  
Medicare Subsidy 0.0 0.0  
Benefit Obligation as of December 31 501.1 492.9 497.5
Change in Fair Value of Plan Assets      
Fair Value of Plan Assets as of January 1 473.8 472.1  
Actual Gain (Loss) on Plan Assets 58.9 30.9  
Company Contributions 8.2 7.2  
Participant Contributions 0.0 0.0  
Benefit Payments (31.8) (36.4)  
Fair Value of Plan Assets as of December 31 509.1 473.8 $ 472.1
Funded (Underfunded) Status as of December 31 8.0 (19.1)  
Benefit Amounts Recognized on the Balance Sheets      
Deferred Charges and Other Noncurrent Assets - Prepaid Benefit Costs 8.4 0.0  
Deferred Credits and Other Noncurrent Liabilities - Accrued Long-term Benefit Liability (0.4) (19.1)  
Funded (Underfunded) Status 8.0 (19.1)  
Components      
Net Actuarial Loss 189.6 215.4  
Prior Service Cost (Credit) 0.0 0.1  
Components of Change in Amounts Included in AOCI and Regulatory Assets      
Actuarial (Gain) Loss During the Year (18.0) 1.5  
Amortization of Actuarial Gain (Loss) (7.8) (8.1)  
Amortization of Prior Service Credit (Cost) (0.1) (0.1)  
Change for the Year $ (25.9) $ (6.7)  
Benefit Plans (Textuals) [Abstract]      
Rate of Compensation Increase [1] 4.95% 4.85%  
Pension Plans [Member] | Ohio Power Co [Member] | Regulatory Assets [Member]      
Recorded as      
Total of Amounts Included in AOCI, Income Tax Expense and Regulatory Assets $ 189.6 $ 215.5  
Pension Plans [Member] | Public Service Co Of Oklahoma [Member]      
Defined Benefit Plan, Accumulated Benefit Obligation $ 259.6 $ 254.2  
Actuarial Assumptions for Benefit Obligations      
Discount Rate 3.65% 4.05%  
Rate of Compensation Increase [1] 4.90% 4.90%  
Actuarial Assumptions for Net Periodic Benefit Costs      
Discount Rate 4.05% 4.30% 4.00%
Expected Return on Plan Assets 6.00% 6.00% 6.00%
Rate of Compensation Increase [2] 4.90% 4.90% 4.80%
Change in Benefit Obligation      
Benefit Obligation as of January 1 $ 266.7 $ 265.4  
Service Cost 6.4 6.2 $ 6.4
Interest Cost 10.7 11.2 10.9
Actuarial (Gain) Loss 10.1 3.1  
Benefit Payments (17.3) (19.2)  
Participant Contributions 0.0 0.0  
Benefit Obligation as of December 31 276.6 266.7 265.4
Change in Fair Value of Plan Assets      
Fair Value of Plan Assets as of January 1 266.0 262.1  
Actual Gain (Loss) on Plan Assets 33.6 17.3  
Company Contributions 5.5 5.8  
Participant Contributions 0.0 0.0  
Benefit Payments (17.3) (19.2)  
Fair Value of Plan Assets as of December 31 287.8 266.0 $ 262.1
Funded (Underfunded) Status as of December 31 11.2 (0.7)  
Benefit Amounts Recognized on the Balance Sheets      
Employee Benefits and Pension Assets - Prepaid Benefit Costs 13.9 1.6  
Other Current Liabilities - Accrued Short-term Benefit Liability (0.2) (0.2)  
Deferred Credits and Other Noncurrent Liabilities - Accrued Long-term Benefit Liability (2.5) (2.1)  
Funded (Underfunded) Status 11.2 (0.7)  
Components      
Net Actuarial Loss 78.8 91.0  
Prior Service Cost (Credit) 0.0 0.0  
Components of Change in Amounts Included in AOCI and Regulatory Assets      
Actuarial (Gain) Loss During the Year (7.9) 1.3  
Amortization of Actuarial Gain (Loss) (4.3) (4.4)  
Amortization of Prior Service Credit (Cost) 0.0 (0.3)  
Change for the Year $ (12.2) $ (3.4)  
Benefit Plans (Textuals) [Abstract]      
Rate of Compensation Increase [1] 4.90% 4.90%  
Pension Plans [Member] | Public Service Co Of Oklahoma [Member] | Regulatory Assets [Member]      
Recorded as      
Total of Amounts Included in AOCI, Income Tax Expense and Regulatory Assets $ 78.8 $ 91.0  
Pension Plans [Member] | Southwestern Electric Power Co [Member]      
Defined Benefit Plan, Accumulated Benefit Obligation $ 291.6 $ 281.5  
Actuarial Assumptions for Benefit Obligations      
Discount Rate 3.65% 4.05%  
Rate of Compensation Increase [1] 4.80% 4.75%  
Actuarial Assumptions for Net Periodic Benefit Costs      
Discount Rate 4.05% 4.30% 4.00%
Expected Return on Plan Assets 6.00% 6.00% 6.00%
Rate of Compensation Increase [2] 4.80% 4.75% 4.80%
Change in Benefit Obligation      
Benefit Obligation as of January 1 $ 296.6 $ 282.8  
Service Cost 8.7 8.1 $ 8.3
Interest Cost 12.3 12.4 11.8
Actuarial (Gain) Loss 16.3 13.8  
Benefit Payments (19.3) (20.5)  
Participant Contributions 0.0 0.0  
Benefit Obligation as of December 31 314.6 296.6 282.8
Change in Fair Value of Plan Assets      
Fair Value of Plan Assets as of January 1 287.3 280.6  
Actual Gain (Loss) on Plan Assets 34.6 18.8  
Company Contributions 9.1 8.4  
Participant Contributions 0.0 0.0  
Benefit Payments (19.3) (20.5)  
Fair Value of Plan Assets as of December 31 311.7 287.3 $ 280.6
Funded (Underfunded) Status as of December 31 (2.9) (9.3)  
Benefit Amounts Recognized on the Balance Sheets      
Deferred Charges and Other Noncurrent Assets - Prepaid Benefit Costs 0.0 0.0  
Other Current Liabilities - Accrued Short-term Benefit Liability (0.2) (0.1)  
Employee Benefits and Pension Obligations - Accrued Long-term Benefit Liability (2.7) (9.2)  
Funded (Underfunded) Status (2.9) (9.3)  
Components      
Net Actuarial Loss 97.4 103.8  
Prior Service Cost (Credit) 0.0 0.1  
Components of Change in Amounts Included in AOCI and Regulatory Assets      
Actuarial (Gain) Loss During the Year (1.5) 11.5  
Amortization of Actuarial Gain (Loss) (4.9) (4.8)  
Amortization of Prior Service Credit (Cost) (0.1) (0.3)  
Change for the Year $ (6.5) $ 6.4  
Benefit Plans (Textuals) [Abstract]      
Rate of Compensation Increase [1] 4.80% 4.75%  
Pension Plans [Member] | Southwestern Electric Power Co [Member] | Income Tax Expense [Member]      
Recorded as      
Total of Amounts Included in AOCI, Income Tax Expense and Regulatory Assets [3] $ 0.0 $ 0.0  
Pension Plans [Member] | Southwestern Electric Power Co [Member] | Regulatory Assets [Member]      
Recorded as      
Total of Amounts Included in AOCI, Income Tax Expense and Regulatory Assets 97.4 103.9  
Pension Plans [Member] | Southwestern Electric Power Co [Member] | Deferred Income Taxes [Member]      
Recorded as      
Total of Amounts Included in AOCI, Income Tax Expense and Regulatory Assets 0.0 0.0  
Pension Plans [Member] | Southwestern Electric Power Co [Member] | Net of Tax AOCI [Member]      
Recorded as      
Total of Amounts Included in AOCI, Income Tax Expense and Regulatory Assets $ 0.0 $ 0.0  
Pension Plans [Member] | Minimum [Member]      
Actuarial Assumptions for Benefit Obligations      
Rate of Compensation Increase 3.50%    
Benefit Plans (Textuals) [Abstract]      
Rate of Compensation Increase 3.50%    
Pension Plans [Member] | Minimum [Member] | AEP Texas Inc. [Member]      
Actuarial Assumptions for Benefit Obligations      
Rate of Compensation Increase 3.50%    
Benefit Plans (Textuals) [Abstract]      
Rate of Compensation Increase 3.50%    
Pension Plans [Member] | Minimum [Member] | Appalachian Power Co [Member]      
Actuarial Assumptions for Benefit Obligations      
Rate of Compensation Increase 3.50%    
Benefit Plans (Textuals) [Abstract]      
Rate of Compensation Increase 3.50%    
Pension Plans [Member] | Minimum [Member] | Indiana Michigan Power Co [Member]      
Actuarial Assumptions for Benefit Obligations      
Rate of Compensation Increase 3.50%    
Benefit Plans (Textuals) [Abstract]      
Rate of Compensation Increase 3.50%    
Pension Plans [Member] | Minimum [Member] | Ohio Power Co [Member]      
Actuarial Assumptions for Benefit Obligations      
Rate of Compensation Increase 3.50%    
Benefit Plans (Textuals) [Abstract]      
Rate of Compensation Increase 3.50%    
Pension Plans [Member] | Minimum [Member] | Public Service Co Of Oklahoma [Member]      
Actuarial Assumptions for Benefit Obligations      
Rate of Compensation Increase 3.50%    
Benefit Plans (Textuals) [Abstract]      
Rate of Compensation Increase 3.50%    
Pension Plans [Member] | Minimum [Member] | Southwestern Electric Power Co [Member]      
Actuarial Assumptions for Benefit Obligations      
Rate of Compensation Increase 3.50%    
Benefit Plans (Textuals) [Abstract]      
Rate of Compensation Increase 3.50%    
Pension Plans [Member] | Maximum [Member]      
Actuarial Assumptions for Benefit Obligations      
Rate of Compensation Increase 12.00%    
Benefit Plans (Textuals) [Abstract]      
Rate of Compensation Increase 12.00%    
Pension Plans [Member] | Maximum [Member] | AEP Texas Inc. [Member]      
Actuarial Assumptions for Benefit Obligations      
Rate of Compensation Increase 12.00%    
Benefit Plans (Textuals) [Abstract]      
Rate of Compensation Increase 12.00%    
Pension Plans [Member] | Maximum [Member] | Appalachian Power Co [Member]      
Actuarial Assumptions for Benefit Obligations      
Rate of Compensation Increase 12.00%    
Benefit Plans (Textuals) [Abstract]      
Rate of Compensation Increase 12.00%    
Pension Plans [Member] | Maximum [Member] | Indiana Michigan Power Co [Member]      
Actuarial Assumptions for Benefit Obligations      
Rate of Compensation Increase 12.00%    
Benefit Plans (Textuals) [Abstract]      
Rate of Compensation Increase 12.00%    
Pension Plans [Member] | Maximum [Member] | Ohio Power Co [Member]      
Actuarial Assumptions for Benefit Obligations      
Rate of Compensation Increase 12.00%    
Benefit Plans (Textuals) [Abstract]      
Rate of Compensation Increase 12.00%    
Pension Plans [Member] | Maximum [Member] | Public Service Co Of Oklahoma [Member]      
Actuarial Assumptions for Benefit Obligations      
Rate of Compensation Increase 12.00%    
Benefit Plans (Textuals) [Abstract]      
Rate of Compensation Increase 12.00%    
Pension Plans [Member] | Maximum [Member] | Southwestern Electric Power Co [Member]      
Actuarial Assumptions for Benefit Obligations      
Rate of Compensation Increase 12.00%    
Benefit Plans (Textuals) [Abstract]      
Rate of Compensation Increase 12.00%    
Other Postretirement Benefit Plans [Member]      
Actuarial Assumptions for Benefit Obligations      
Discount Rate 3.60% 4.10%  
Actuarial Assumptions for Net Periodic Benefit Costs      
Discount Rate 4.10% 4.30% 4.00%
Expected Return on Plan Assets 6.75% 7.00% 6.75%
Health Care Trend Rates      
Initial 6.50% 7.00%  
Ultimate 5.00% 5.00%  
Year Ultimate Reached 2024 2024  
Effect of a 1% Change in Assumed Health Care Cost Trend Rates for the OPEB Health Care Plans      
Effect of 1% Increase on Total Service and Interest Cost Components of Net Periodic Postretirement Health Care Benefit Cost $ 2.5    
Effect of 1% Decrease on Total Service and Interest Cost Components of Net Periodic Postretirement Health Care Benefit Cost (2.0)    
Effect of 1% Increase on the Health Care Component of the Accumulated Postretirement Benefit Obligation 45.4    
Effect of 1% Decrease on the Health Care Component of the Accumulated Postretirement Benefit Obligation (39.6)    
Change in Benefit Obligation      
Benefit Obligation as of January 1 1,447.4 $ 1,450.6  
Service Cost 11.2 10.2 $ 12.2
Interest Cost 59.3 60.9 56.8
Actuarial (Gain) Loss (97.5) 17.3  
Benefit Payments (128.6) (130.2)  
Participant Contributions 39.5 37.8  
Medicare Subsidy 0.7 0.8  
Benefit Obligation as of December 31 1,332.0 1,447.4 1,450.6
Change in Fair Value of Plan Assets      
Fair Value of Plan Assets as of January 1 1,545.9 1,577.4  
Actual Gain (Loss) on Plan Assets 271.6 56.0  
Company Contributions 4.1 4.9  
Participant Contributions 39.5 37.8  
Benefit Payments (128.6) (130.2)  
Fair Value of Plan Assets as of December 31 1,732.5 1,545.9 $ 1,577.4
Funded (Underfunded) Status as of December 31 400.5 98.5  
Benefit Amounts Recognized on the Balance Sheets      
Deferred Charges and Other Noncurrent Assets - Prepaid Benefit Costs 463.0 154.5  
Other Current Liabilities - Accrued Short-term Benefit Liability (3.2) (3.0)  
Employee Benefits and Pension Obligations - Accrued Long-term Benefit Liability (59.3) (53.0)  
Funded (Underfunded) Status 400.5 98.5  
Components      
Net Actuarial Loss 309.9 614.4  
Prior Service Cost (Credit) (416.3) (485.4)  
Components of Change in Amounts Included in AOCI and Regulatory Assets      
Actuarial (Gain) Loss During the Year (267.8) 68.4  
Amortization of Actuarial Gain (Loss) (36.7) (31.4)  
Amortization of Prior Service Credit (Cost) 69.1 69.0  
Change for the Year (235.4) 106.0  
Other Postretirement Benefit Plans [Member] | Income Tax Expense [Member]      
Recorded as      
Total of Amounts Included in AOCI, Income Tax Expense and Regulatory Assets [3] (3.4) 0.0  
Other Postretirement Benefit Plans [Member] | Regulatory Assets [Member]      
Recorded as      
Total of Amounts Included in AOCI, Income Tax Expense and Regulatory Assets (82.4) 90.4  
Other Postretirement Benefit Plans [Member] | Deferred Income Taxes [Member]      
Recorded as      
Total of Amounts Included in AOCI, Income Tax Expense and Regulatory Assets (5.0) 13.5  
Other Postretirement Benefit Plans [Member] | Net of Tax AOCI [Member]      
Recorded as      
Total of Amounts Included in AOCI, Income Tax Expense and Regulatory Assets $ (15.6) $ 25.1  
Other Postretirement Benefit Plans [Member] | AEP Texas Inc. [Member]      
Actuarial Assumptions for Benefit Obligations      
Discount Rate 3.60% 4.10%  
Actuarial Assumptions for Net Periodic Benefit Costs      
Discount Rate 4.10% 4.30% 4.00%
Expected Return on Plan Assets 6.75% 7.00% 6.75%
Health Care Trend Rates      
Initial 6.50% 7.00%  
Ultimate 5.00% 5.00%  
Year Ultimate Reached 2024 2024  
Effect of a 1% Change in Assumed Health Care Cost Trend Rates for the OPEB Health Care Plans      
Effect of 1% Increase on Total Service and Interest Cost Components of Net Periodic Postretirement Health Care Benefit Cost $ 0.1    
Effect of 1% Decrease on Total Service and Interest Cost Components of Net Periodic Postretirement Health Care Benefit Cost (0.1)    
Effect of 1% Increase on the Health Care Component of the Accumulated Postretirement Benefit Obligation 2.6    
Effect of 1% Decrease on the Health Care Component of the Accumulated Postretirement Benefit Obligation (2.4)    
Change in Benefit Obligation      
Benefit Obligation as of January 1 120.4 $ 122.0  
Transfer of CSW Energy, Inc. Benefit Obligation 0.0 (0.4)  
Service Cost 0.9 0.7 $ 0.8
Interest Cost 4.9 5.1 4.8
Actuarial (Gain) Loss (11.9) 0.8  
Benefit Payments (10.8) (11.4)  
Participant Contributions 3.6 3.5  
Medicare Subsidy 0.0 0.1  
Benefit Obligation as of December 31 107.1 120.4 122.0
Change in Fair Value of Plan Assets      
Fair Value of Plan Assets as of January 1 134.1 138.6  
Transfer of CSW Energy, Inc. Plan Assets 0.0 (0.4)  
Actual Gain (Loss) on Plan Assets 20.4 3.8  
Company Contributions 0.0 0.0  
Participant Contributions 3.6 3.5  
Benefit Payments (10.8) (11.4)  
Fair Value of Plan Assets as of December 31 147.3 134.1 $ 138.6
Funded (Underfunded) Status as of December 31 40.2 13.7  
Benefit Amounts Recognized on the Balance Sheets      
Deferred Charges and Other Noncurrent Assets - Prepaid Benefit Costs 40.2 13.7  
Other Current Liabilities - Accrued Short-term Benefit Liability 0.0 0.0  
Deferred Credits and Other Noncurrent Liabilities - Accrued Long-term Benefit Liability 0.0 0.0  
Funded (Underfunded) Status 40.2 13.7  
Components      
Net Actuarial Loss 23.9 50.7  
Prior Service Cost (Credit) (35.4) (41.2)  
Components of Change in Amounts Included in AOCI and Regulatory Assets      
Actuarial (Gain) Loss During the Year (23.6) 6.4  
Amortization of Actuarial Gain (Loss) (3.2) (2.8)  
Amortization of Prior Service Credit (Cost) 5.8 6.0  
Change for the Year (21.0) 9.6  
Other Postretirement Benefit Plans [Member] | AEP Texas Inc. [Member] | Income Tax Expense [Member]      
Recorded as      
Total of Amounts Included in AOCI, Income Tax Expense and Regulatory Assets [3] (0.2) 0.0  
Other Postretirement Benefit Plans [Member] | AEP Texas Inc. [Member] | Regulatory Assets [Member]      
Recorded as      
Total of Amounts Included in AOCI, Income Tax Expense and Regulatory Assets (10.2) 9.7  
Other Postretirement Benefit Plans [Member] | AEP Texas Inc. [Member] | Deferred Income Taxes [Member]      
Recorded as      
Total of Amounts Included in AOCI, Income Tax Expense and Regulatory Assets (0.3) (0.1)  
Other Postretirement Benefit Plans [Member] | AEP Texas Inc. [Member] | Net of Tax AOCI [Member]      
Recorded as      
Total of Amounts Included in AOCI, Income Tax Expense and Regulatory Assets $ (0.8) $ (0.1)  
Other Postretirement Benefit Plans [Member] | Appalachian Power Co [Member]      
Actuarial Assumptions for Benefit Obligations      
Discount Rate 3.60% 4.10%  
Actuarial Assumptions for Net Periodic Benefit Costs      
Discount Rate 4.10% 4.30% 4.00%
Expected Return on Plan Assets 6.75% 7.00% 6.75%
Health Care Trend Rates      
Initial 6.50% 7.00%  
Ultimate 5.00% 5.00%  
Year Ultimate Reached 2024 2024  
Effect of a 1% Change in Assumed Health Care Cost Trend Rates for the OPEB Health Care Plans      
Effect of 1% Increase on Total Service and Interest Cost Components of Net Periodic Postretirement Health Care Benefit Cost $ 0.5    
Effect of 1% Decrease on Total Service and Interest Cost Components of Net Periodic Postretirement Health Care Benefit Cost (0.4)    
Effect of 1% Increase on the Health Care Component of the Accumulated Postretirement Benefit Obligation 10.8    
Effect of 1% Decrease on the Health Care Component of the Accumulated Postretirement Benefit Obligation (9.1)    
Change in Benefit Obligation      
Benefit Obligation as of January 1 255.6 $ 262.2  
Service Cost 1.1 1.0 $ 1.1
Interest Cost 10.6 10.8 10.3
Actuarial (Gain) Loss (13.4) (0.2)  
Benefit Payments (24.3) (24.8)  
Participant Contributions 6.7 6.4  
Medicare Subsidy 0.2 0.2  
Benefit Obligation as of December 31 236.5 255.6 262.2
Change in Fair Value of Plan Assets      
Fair Value of Plan Assets as of January 1 246.9 256.7  
Actual Gain (Loss) on Plan Assets 41.6 5.9  
Company Contributions 2.5 2.7  
Participant Contributions 6.7 6.4  
Benefit Payments (24.3) (24.8)  
Fair Value of Plan Assets as of December 31 273.4 246.9 $ 256.7
Funded (Underfunded) Status as of December 31 36.9 (8.7)  
Benefit Amounts Recognized on the Balance Sheets      
Deferred Charges and Other Noncurrent Assets - Prepaid Benefit Costs 74.6 25.2  
Other Current Liabilities - Accrued Short-term Benefit Liability (2.5) (2.4)  
Employee Benefits and Pension Obligations - Accrued Long-term Benefit Liability (35.2) (31.5)  
Funded (Underfunded) Status 36.9 (8.7)  
Components      
Net Actuarial Loss 48.0 92.9  
Prior Service Cost (Credit) (60.4) (70.5)  
Components of Change in Amounts Included in AOCI and Regulatory Assets      
Actuarial (Gain) Loss During the Year (38.6) 11.4  
Amortization of Actuarial Gain (Loss) (6.3) (5.4)  
Amortization of Prior Service Credit (Cost) 10.1 10.1  
Change for the Year (34.8) 16.1  
Other Postretirement Benefit Plans [Member] | Appalachian Power Co [Member] | Income Tax Expense [Member]      
Recorded as      
Total of Amounts Included in AOCI, Income Tax Expense and Regulatory Assets [3] (0.2) 0.0  
Other Postretirement Benefit Plans [Member] | Appalachian Power Co [Member] | Regulatory Assets [Member]      
Recorded as      
Total of Amounts Included in AOCI, Income Tax Expense and Regulatory Assets (11.1) 7.7  
Other Postretirement Benefit Plans [Member] | Appalachian Power Co [Member] | Deferred Income Taxes [Member]      
Recorded as      
Total of Amounts Included in AOCI, Income Tax Expense and Regulatory Assets (0.3) 5.1  
Other Postretirement Benefit Plans [Member] | Appalachian Power Co [Member] | Net of Tax AOCI [Member]      
Recorded as      
Total of Amounts Included in AOCI, Income Tax Expense and Regulatory Assets $ (0.8) $ 9.6  
Other Postretirement Benefit Plans [Member] | Indiana Michigan Power Co [Member]      
Actuarial Assumptions for Benefit Obligations      
Discount Rate 3.60% 4.10%  
Actuarial Assumptions for Net Periodic Benefit Costs      
Discount Rate 4.10% 4.30% 4.00%
Expected Return on Plan Assets 6.75% 7.00% 6.75%
Health Care Trend Rates      
Initial 6.50% 7.00%  
Ultimate 5.00% 5.00%  
Year Ultimate Reached 2024 2024  
Effect of a 1% Change in Assumed Health Care Cost Trend Rates for the OPEB Health Care Plans      
Effect of 1% Increase on Total Service and Interest Cost Components of Net Periodic Postretirement Health Care Benefit Cost $ 0.2    
Effect of 1% Decrease on Total Service and Interest Cost Components of Net Periodic Postretirement Health Care Benefit Cost (0.2)    
Effect of 1% Increase on the Health Care Component of the Accumulated Postretirement Benefit Obligation 3.7    
Effect of 1% Decrease on the Health Care Component of the Accumulated Postretirement Benefit Obligation (3.4)    
Change in Benefit Obligation      
Benefit Obligation as of January 1 167.6 $ 166.3  
Service Cost 1.6 1.5 $ 1.6
Interest Cost 6.9 7.0 6.4
Actuarial (Gain) Loss (12.0) 3.8  
Benefit Payments (15.6) (15.7)  
Participant Contributions 4.9 4.6  
Medicare Subsidy 0.1 0.1  
Benefit Obligation as of December 31 153.5 167.6 166.3
Change in Fair Value of Plan Assets      
Fair Value of Plan Assets as of January 1 186.6 189.0  
Actual Gain (Loss) on Plan Assets 35.2 8.7  
Company Contributions 0.0 0.0  
Participant Contributions 4.9 4.6  
Benefit Payments (15.6) (15.7)  
Fair Value of Plan Assets as of December 31 211.1 186.6 $ 189.0
Funded (Underfunded) Status as of December 31 57.6 19.0  
Benefit Amounts Recognized on the Balance Sheets      
Deferred Charges and Other Noncurrent Assets - Prepaid Benefit Costs 57.6 19.0  
Deferred Credits and Other Noncurrent Liabilities - Accrued Long-term Benefit Liability 0.0 0.0  
Funded (Underfunded) Status 57.6 19.0  
Components      
Net Actuarial Loss 42.0 81.3  
Prior Service Cost (Credit) (56.9) (66.3)  
Components of Change in Amounts Included in AOCI and Regulatory Assets      
Actuarial (Gain) Loss During the Year (34.9) 7.9  
Amortization of Actuarial Gain (Loss) (4.4) (3.7)  
Amortization of Prior Service Credit (Cost) 9.4 9.4  
Change for the Year (29.9) 13.6  
Other Postretirement Benefit Plans [Member] | Indiana Michigan Power Co [Member] | Income Tax Expense [Member]      
Recorded as      
Total of Amounts Included in AOCI, Income Tax Expense and Regulatory Assets [3] (0.1) 0.0  
Other Postretirement Benefit Plans [Member] | Indiana Michigan Power Co [Member] | Regulatory Assets [Member]      
Recorded as      
Total of Amounts Included in AOCI, Income Tax Expense and Regulatory Assets (14.0) 13.7  
Other Postretirement Benefit Plans [Member] | Indiana Michigan Power Co [Member] | Deferred Income Taxes [Member]      
Recorded as      
Total of Amounts Included in AOCI, Income Tax Expense and Regulatory Assets (0.2) 0.5  
Other Postretirement Benefit Plans [Member] | Indiana Michigan Power Co [Member] | Net of Tax AOCI [Member]      
Recorded as      
Total of Amounts Included in AOCI, Income Tax Expense and Regulatory Assets $ (0.6) $ 0.8  
Other Postretirement Benefit Plans [Member] | Ohio Power Co [Member]      
Actuarial Assumptions for Benefit Obligations      
Discount Rate 3.60% 4.10%  
Actuarial Assumptions for Net Periodic Benefit Costs      
Discount Rate 4.10% 4.30% 4.00%
Expected Return on Plan Assets 6.75% 7.00% 6.75%
Health Care Trend Rates      
Initial 6.50% 7.00%  
Ultimate 5.00% 5.00%  
Year Ultimate Reached 2024 2024  
Effect of a 1% Change in Assumed Health Care Cost Trend Rates for the OPEB Health Care Plans      
Effect of 1% Increase on Total Service and Interest Cost Components of Net Periodic Postretirement Health Care Benefit Cost $ 0.2    
Effect of 1% Decrease on Total Service and Interest Cost Components of Net Periodic Postretirement Health Care Benefit Cost (0.2)    
Effect of 1% Increase on the Health Care Component of the Accumulated Postretirement Benefit Obligation 3.5    
Effect of 1% Decrease on the Health Care Component of the Accumulated Postretirement Benefit Obligation (3.2)    
Change in Benefit Obligation      
Benefit Obligation as of January 1 164.0 $ 168.6  
Service Cost 0.9 0.8 $ 0.9
Interest Cost 6.7 7.0 6.4
Actuarial (Gain) Loss (16.6) (1.0)  
Benefit Payments (15.5) (16.2)  
Participant Contributions 4.7 4.7  
Medicare Subsidy 0.1 0.1  
Benefit Obligation as of December 31 144.3 164.0 168.6
Change in Fair Value of Plan Assets      
Fair Value of Plan Assets as of January 1 182.6 191.6  
Actual Gain (Loss) on Plan Assets 26.7 2.5  
Company Contributions 0.0 0.0  
Participant Contributions 4.7 4.7  
Benefit Payments (15.5) (16.2)  
Fair Value of Plan Assets as of December 31 198.5 182.6 $ 191.6
Funded (Underfunded) Status as of December 31 54.2 18.6  
Benefit Amounts Recognized on the Balance Sheets      
Deferred Charges and Other Noncurrent Assets - Prepaid Benefit Costs 54.2 18.6  
Deferred Credits and Other Noncurrent Liabilities - Accrued Long-term Benefit Liability 0.0 0.0  
Funded (Underfunded) Status 54.2 18.6  
Components      
Net Actuarial Loss 22.6 58.2  
Prior Service Cost (Credit) (41.6) (48.5)  
Components of Change in Amounts Included in AOCI and Regulatory Assets      
Actuarial (Gain) Loss During the Year (31.3) 9.4  
Amortization of Actuarial Gain (Loss) (4.3) (3.8)  
Amortization of Prior Service Credit (Cost) 6.9 6.9  
Change for the Year (28.7) 12.5  
Other Postretirement Benefit Plans [Member] | Ohio Power Co [Member] | Regulatory Assets [Member]      
Recorded as      
Total of Amounts Included in AOCI, Income Tax Expense and Regulatory Assets $ (19.0) $ 9.7  
Other Postretirement Benefit Plans [Member] | Public Service Co Of Oklahoma [Member]      
Actuarial Assumptions for Benefit Obligations      
Discount Rate 3.60% 4.10%  
Actuarial Assumptions for Net Periodic Benefit Costs      
Discount Rate 4.10% 4.30% 4.00%
Expected Return on Plan Assets 6.75% 7.00% 6.75%
Health Care Trend Rates      
Initial 6.50% 7.00%  
Ultimate 5.00% 5.00%  
Year Ultimate Reached 2024 2024  
Effect of a 1% Change in Assumed Health Care Cost Trend Rates for the OPEB Health Care Plans      
Effect of 1% Increase on Total Service and Interest Cost Components of Net Periodic Postretirement Health Care Benefit Cost $ 0.1    
Effect of 1% Decrease on Total Service and Interest Cost Components of Net Periodic Postretirement Health Care Benefit Cost (0.1)    
Effect of 1% Increase on the Health Care Component of the Accumulated Postretirement Benefit Obligation 1.7    
Effect of 1% Decrease on the Health Care Component of the Accumulated Postretirement Benefit Obligation (1.5)    
Change in Benefit Obligation      
Benefit Obligation as of January 1 77.6 $ 77.7  
Service Cost 0.7 0.6 $ 0.7
Interest Cost 3.2 3.3 3.0
Actuarial (Gain) Loss (7.5) 1.0  
Benefit Payments (6.9) (7.2)  
Participant Contributions 2.3 2.2  
Benefit Obligation as of December 31 69.4 77.6 77.7
Change in Fair Value of Plan Assets      
Fair Value of Plan Assets as of January 1 86.4 88.3  
Actual Gain (Loss) on Plan Assets 13.7 3.1  
Company Contributions 0.0 0.0  
Participant Contributions 2.3 2.2  
Benefit Payments (6.9) (7.2)  
Fair Value of Plan Assets as of December 31 95.5 86.4 $ 88.3
Funded (Underfunded) Status as of December 31 26.1 8.8  
Benefit Amounts Recognized on the Balance Sheets      
Employee Benefits and Pension Assets - Prepaid Benefit Costs 26.1 8.8  
Other Current Liabilities - Accrued Short-term Benefit Liability 0.0 0.0  
Deferred Credits and Other Noncurrent Liabilities - Accrued Long-term Benefit Liability 0.0 0.0  
Funded (Underfunded) Status 26.1 8.8  
Components      
Net Actuarial Loss 19.8 37.3  
Prior Service Cost (Credit) (25.9) (30.2)  
Components of Change in Amounts Included in AOCI and Regulatory Assets      
Actuarial (Gain) Loss During the Year (15.5) 3.9  
Amortization of Actuarial Gain (Loss) (2.0) (1.8)  
Amortization of Prior Service Credit (Cost) 4.3 4.3  
Change for the Year (13.2) 6.4  
Other Postretirement Benefit Plans [Member] | Public Service Co Of Oklahoma [Member] | Regulatory Assets [Member]      
Recorded as      
Total of Amounts Included in AOCI, Income Tax Expense and Regulatory Assets $ (6.1) $ 7.1  
Other Postretirement Benefit Plans [Member] | Southwestern Electric Power Co [Member]      
Actuarial Assumptions for Benefit Obligations      
Discount Rate 3.60% 4.10%  
Actuarial Assumptions for Net Periodic Benefit Costs      
Discount Rate 4.10% 4.30% 4.00%
Expected Return on Plan Assets 6.75% 7.00% 6.75%
Health Care Trend Rates      
Initial 6.50% 7.00%  
Ultimate 5.00% 5.00%  
Year Ultimate Reached 2024 2024  
Effect of a 1% Change in Assumed Health Care Cost Trend Rates for the OPEB Health Care Plans      
Effect of 1% Increase on Total Service and Interest Cost Components of Net Periodic Postretirement Health Care Benefit Cost $ 0.1    
Effect of 1% Decrease on Total Service and Interest Cost Components of Net Periodic Postretirement Health Care Benefit Cost (0.1)    
Effect of 1% Increase on the Health Care Component of the Accumulated Postretirement Benefit Obligation 1.9    
Effect of 1% Decrease on the Health Care Component of the Accumulated Postretirement Benefit Obligation (1.8)    
Change in Benefit Obligation      
Benefit Obligation as of January 1 86.9 $ 86.1  
Service Cost 0.9 0.8 $ 0.8
Interest Cost 3.6 3.6 3.4
Actuarial (Gain) Loss (6.2) 1.5  
Benefit Payments (7.4) (7.5)  
Participant Contributions 2.5 2.4  
Benefit Obligation as of December 31 80.3 86.9 86.1
Change in Fair Value of Plan Assets      
Fair Value of Plan Assets as of January 1 96.8 97.8  
Actual Gain (Loss) on Plan Assets 18.5 4.1  
Company Contributions 0.0 0.0  
Participant Contributions 2.5 2.4  
Benefit Payments (7.4) (7.5)  
Fair Value of Plan Assets as of December 31 110.4 96.8 $ 97.8
Funded (Underfunded) Status as of December 31 30.1 9.9  
Benefit Amounts Recognized on the Balance Sheets      
Deferred Charges and Other Noncurrent Assets - Prepaid Benefit Costs 30.1 9.9  
Other Current Liabilities - Accrued Short-term Benefit Liability 0.0 0.0  
Employee Benefits and Pension Obligations - Accrued Long-term Benefit Liability 0.0 0.0  
Funded (Underfunded) Status 30.1 9.9  
Components      
Net Actuarial Loss 24.7 45.4  
Prior Service Cost (Credit) (31.4) (36.6)  
Components of Change in Amounts Included in AOCI and Regulatory Assets      
Actuarial (Gain) Loss During the Year (18.4) 4.0  
Amortization of Actuarial Gain (Loss) (2.3) (1.9)  
Amortization of Prior Service Credit (Cost) 5.2 5.0  
Change for the Year (15.5) 7.1  
Other Postretirement Benefit Plans [Member] | Southwestern Electric Power Co [Member] | Income Tax Expense [Member]      
Recorded as      
Total of Amounts Included in AOCI, Income Tax Expense and Regulatory Assets [3] (0.4) 0.0  
Other Postretirement Benefit Plans [Member] | Southwestern Electric Power Co [Member] | Regulatory Assets [Member]      
Recorded as      
Total of Amounts Included in AOCI, Income Tax Expense and Regulatory Assets (3.7) 5.7  
Other Postretirement Benefit Plans [Member] | Southwestern Electric Power Co [Member] | Deferred Income Taxes [Member]      
Recorded as      
Total of Amounts Included in AOCI, Income Tax Expense and Regulatory Assets (0.6) 1.1  
Other Postretirement Benefit Plans [Member] | Southwestern Electric Power Co [Member] | Net of Tax AOCI [Member]      
Recorded as      
Total of Amounts Included in AOCI, Income Tax Expense and Regulatory Assets $ (2.0) $ 2.0  
[1] Rates are for base pay only. In addition, an amount is added to reflect target incentive compensation for exempt employees and overtime and incentive pay for nonexempt employees.
[2] Rates are for base pay only. In addition, an amount is added to reflect target incentive compensation for exempt employees and overtime and incentive pay for nonexempt employees.
[3] Amounts relate to the re-measurement of Deferred Income Taxes as a result of Tax Reform. In accordance with the accounting guidance for “Income Taxes”, re-measurement of Deferred Income Taxes related to AOCI must flow through the statement of income.