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Comprehensive Income (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2016
Sep. 30, 2015
Sep. 30, 2016
Sep. 30, 2015
Changes in Accumulated Other Comprehensive Income (Loss) by Component        
Beginning Balance in AOCI $ (117.9) $ (102.5) $ (127.1) $ (103.1)
Change in Fair Value Recognized in AOCI (26.2) (4.3) (16.0) (2.1)
Reclassifications From Accumulated Other Comprehensive Income [Abstract]        
Other Revenues 45.2 30.2 134.0 90.2
Purchased Electricity for Resale 774.0 730.8 2,134.6 2,050.0
Interest Expense 225.3 220.2 667.2 658.1
Reclassifications from AOCI, before Income Tax (Expense) Credit (2.8) (4.9) (4.2) (13.5)
Income Tax (Expense) Credit 534.5 (275.6) 134.0 (827.1)
Reclassifications from AOCI, Net of Income Tax (Expense) Credit (1.7) (2.0) (2.7) (8.7)
Net Current Period Other Comprehensive Income (27.9) (6.3) (18.7) (10.8)
Pension and OPEB Adjustment Related to Mitchell Plant       5.1
Ending Balance in AOCI (145.8) (108.8) (145.8) (108.8)
Securities Available for Sale [Member]        
Changes in Accumulated Other Comprehensive Income (Loss) by Component        
Beginning Balance in AOCI 8.3 8.0 7.1 7.7
Change in Fair Value Recognized in AOCI 0.5 (1.3) 1.7 (1.0)
Reclassifications From Accumulated Other Comprehensive Income [Abstract]        
Reclassifications from AOCI, before Income Tax (Expense) Credit 0.0 0.0 0.0 0.0
Reclassifications from AOCI, Net of Income Tax (Expense) Credit 0.0 0.0 0.0 0.0
Net Current Period Other Comprehensive Income 0.5 (1.3) 1.7 (1.0)
Pension and OPEB Adjustment Related to Mitchell Plant       0.0
Ending Balance in AOCI 8.8 6.7 8.8 6.7
Pension and OPEB [Member]        
Changes in Accumulated Other Comprehensive Income (Loss) by Component        
Beginning Balance in AOCI (111.6) (87.6) (111.8) (93.3)
Change in Fair Value Recognized in AOCI 0.0 0.0 0.0 0.0
Reclassifications From Accumulated Other Comprehensive Income [Abstract]        
Reclassifications from AOCI, before Income Tax (Expense) Credit 0.2 0.5 0.6 1.4
Reclassifications from AOCI, Net of Income Tax (Expense) Credit 0.2 0.3 0.4 0.9
Net Current Period Other Comprehensive Income 0.2 0.3 0.4 0.9
Pension and OPEB Adjustment Related to Mitchell Plant       5.1
Ending Balance in AOCI (111.4) (87.3) (111.4) (87.3)
Commodity [Member] | Cash Flow Hedges [Member]        
Changes in Accumulated Other Comprehensive Income (Loss) by Component        
Beginning Balance in AOCI 1.9 (5.2) (5.2) 1.6
Change in Fair Value Recognized in AOCI (26.7) (3.3) (17.7) (2.0)
Reclassifications From Accumulated Other Comprehensive Income [Abstract]        
Reclassifications from AOCI, before Income Tax (Expense) Credit (3.6) (5.2) (6.5) (15.9)
Reclassifications from AOCI, Net of Income Tax (Expense) Credit (2.3) (2.2) (4.2) (10.3)
Net Current Period Other Comprehensive Income (29.0) (5.5) (21.9) (12.3)
Pension and OPEB Adjustment Related to Mitchell Plant       0.0
Ending Balance in AOCI (27.1) (10.7) (27.1) (10.7)
Interest Rate and Foreign Currency [Member] | Cash Flow Hedges [Member]        
Changes in Accumulated Other Comprehensive Income (Loss) by Component        
Beginning Balance in AOCI (16.5) (17.7) (17.2) (19.1)
Change in Fair Value Recognized in AOCI 0.0 0.3 0.0 0.9
Reclassifications From Accumulated Other Comprehensive Income [Abstract]        
Reclassifications from AOCI, before Income Tax (Expense) Credit 0.6 (0.2) 1.7 1.0
Reclassifications from AOCI, Net of Income Tax (Expense) Credit 0.4 (0.1) 1.1 0.7
Net Current Period Other Comprehensive Income 0.4 0.2 1.1 1.6
Pension and OPEB Adjustment Related to Mitchell Plant       0.0
Ending Balance in AOCI (16.1) (17.5) (16.1) (17.5)
Reclassifications from Accumulated Other Comprehensive Income [Member]        
Reclassifications From Accumulated Other Comprehensive Income [Abstract]        
Other Revenues (5.4) (19.5) (20.7) (36.3)
Purchased Electricity for Resale 1.8 14.3 14.2 20.4
Interest Expense 0.6 (0.2) 1.7 1.0
Prior Service Cost (Credit) (4.8) (4.8) (14.6) (14.6)
Actuarial (Gains)/Losses 5.0 5.3 15.2 16.0
Income Tax (Expense) Credit (1.1) (2.9) (1.5) (4.8)
Reclassifications from Accumulated Other Comprehensive Income [Member] | Securities Available for Sale [Member]        
Reclassifications From Accumulated Other Comprehensive Income [Abstract]        
Other Revenues 0.0 0.0 0.0 0.0
Purchased Electricity for Resale 0.0 0.0 0.0 0.0
Interest Expense 0.0 0.0 0.0 0.0
Prior Service Cost (Credit) 0.0 0.0 0.0 0.0
Actuarial (Gains)/Losses 0.0 0.0 0.0 0.0
Income Tax (Expense) Credit 0.0 0.0 0.0 0.0
Reclassifications from Accumulated Other Comprehensive Income [Member] | Pension and OPEB [Member]        
Reclassifications From Accumulated Other Comprehensive Income [Abstract]        
Other Revenues 0.0 0.0 0.0 0.0
Purchased Electricity for Resale 0.0 0.0 0.0 0.0
Interest Expense 0.0 0.0 0.0 0.0
Prior Service Cost (Credit) (4.8) (4.8) (14.6) (14.6)
Actuarial (Gains)/Losses 5.0 5.3 15.2 16.0
Income Tax (Expense) Credit 0.0 0.2 0.2 0.5
Reclassifications from Accumulated Other Comprehensive Income [Member] | Commodity [Member] | Cash Flow Hedges [Member]        
Reclassifications From Accumulated Other Comprehensive Income [Abstract]        
Other Revenues (5.4) (19.5) (20.7) (36.3)
Purchased Electricity for Resale 1.8 14.3 14.2 20.4
Interest Expense 0.0 0.0 0.0 0.0
Prior Service Cost (Credit) 0.0 0.0 0.0 0.0
Actuarial (Gains)/Losses 0.0 0.0 0.0 0.0
Income Tax (Expense) Credit (1.3) (3.0) (2.3) (5.6)
Reclassifications from Accumulated Other Comprehensive Income [Member] | Interest Rate and Foreign Currency [Member] | Cash Flow Hedges [Member]        
Reclassifications From Accumulated Other Comprehensive Income [Abstract]        
Other Revenues 0.0 0.0 0.0 0.0
Purchased Electricity for Resale 0.0 0.0 0.0 0.0
Interest Expense 0.6 (0.2) 1.7 1.0
Prior Service Cost (Credit) 0.0 0.0 0.0 0.0
Actuarial (Gains)/Losses 0.0 0.0 0.0 0.0
Income Tax (Expense) Credit 0.2 (0.1) 0.6 0.3
Appalachian Power Co [Member]        
Changes in Accumulated Other Comprehensive Income (Loss) by Component        
Beginning Balance in AOCI (3.9) 4.2 (2.8) 5.0
Change in Fair Value Recognized in AOCI 0.0 0.0 0.0 0.0
Reclassifications From Accumulated Other Comprehensive Income [Abstract]        
Other Revenues 2.8 2.9 9.4 7.9
Purchased Electricity for Resale 69.2 80.5 240.9 258.9
Interest Expense 46.4 46.6 140.7 145.6
Reclassifications from AOCI, before Income Tax (Expense) Credit (0.7) (1.0) (2.4) (2.2)
Income Tax (Expense) Credit (58.7) (40.5) (172.7) (168.4)
Reclassifications from AOCI, Net of Income Tax (Expense) Credit (0.5) (0.7) (1.6) (1.5)
Net Current Period Other Comprehensive Income (0.5) (0.7) (1.6) (1.5)
Ending Balance in AOCI (4.4) 3.5 (4.4) 3.5
Appalachian Power Co [Member] | Pension and OPEB [Member]        
Changes in Accumulated Other Comprehensive Income (Loss) by Component        
Beginning Balance in AOCI (7.1) 0.2 (6.4) 1.1
Change in Fair Value Recognized in AOCI 0.0 0.0 0.0 0.0
Reclassifications From Accumulated Other Comprehensive Income [Abstract]        
Reclassifications from AOCI, before Income Tax (Expense) Credit (0.5) (0.7) (1.6) (2.1)
Reclassifications from AOCI, Net of Income Tax (Expense) Credit (0.3) (0.5) (1.0) (1.4)
Net Current Period Other Comprehensive Income (0.3) (0.5) (1.0) (1.4)
Ending Balance in AOCI (7.4) (0.3) (7.4) (0.3)
Appalachian Power Co [Member] | Interest Rate and Foreign Currency [Member] | Cash Flow Hedges [Member]        
Changes in Accumulated Other Comprehensive Income (Loss) by Component        
Beginning Balance in AOCI 3.2 4.0 3.6 3.9
Change in Fair Value Recognized in AOCI 0.0 0.0 0.0 0.0
Reclassifications From Accumulated Other Comprehensive Income [Abstract]        
Reclassifications from AOCI, before Income Tax (Expense) Credit (0.2) (0.3) (0.8) (0.1)
Reclassifications from AOCI, Net of Income Tax (Expense) Credit (0.2) (0.2) (0.6) (0.1)
Net Current Period Other Comprehensive Income (0.2) (0.2) (0.6) (0.1)
Ending Balance in AOCI 3.0 3.8 3.0 3.8
Appalachian Power Co [Member] | Reclassifications from Accumulated Other Comprehensive Income [Member]        
Reclassifications From Accumulated Other Comprehensive Income [Abstract]        
Interest Expense (0.2) (0.3) (0.8) (0.1)
Prior Service Cost (Credit) (1.2) (1.2) (3.8) (3.8)
Actuarial (Gains)/Losses 0.7 0.5 2.2 1.7
Income Tax (Expense) Credit (0.2) (0.3) (0.8) (0.7)
Appalachian Power Co [Member] | Reclassifications from Accumulated Other Comprehensive Income [Member] | Pension and OPEB [Member]        
Reclassifications From Accumulated Other Comprehensive Income [Abstract]        
Interest Expense 0.0 0.0 0.0 0.0
Prior Service Cost (Credit) (1.2) (1.2) (3.8) (3.8)
Actuarial (Gains)/Losses 0.7 0.5 2.2 1.7
Income Tax (Expense) Credit (0.2) (0.2) (0.6) (0.7)
Appalachian Power Co [Member] | Reclassifications from Accumulated Other Comprehensive Income [Member] | Interest Rate and Foreign Currency [Member] | Cash Flow Hedges [Member]        
Reclassifications From Accumulated Other Comprehensive Income [Abstract]        
Interest Expense (0.2) (0.3) (0.8) (0.1)
Prior Service Cost (Credit) 0.0 0.0 0.0 0.0
Actuarial (Gains)/Losses 0.0 0.0 0.0 0.0
Income Tax (Expense) Credit 0.0 (0.1) (0.2) 0.0
Indiana Michigan Power Co [Member]        
Changes in Accumulated Other Comprehensive Income (Loss) by Component        
Beginning Balance in AOCI (16.0) (13.8) (16.7) (14.3)
Change in Fair Value Recognized in AOCI 0.0 0.0 0.0 0.0
Reclassifications From Accumulated Other Comprehensive Income [Abstract]        
Other Revenues 3.4 0.8 13.2 2.6
Purchased Electricity for Resale 43.7 41.5 134.3 147.7
Interest Expense 26.7 23.1 76.3 68.9
Reclassifications from AOCI, before Income Tax (Expense) Credit 0.5 0.4 1.5 1.2
Income Tax (Expense) Credit (35.1) (27.7) (84.3) (86.7)
Reclassifications from AOCI, Net of Income Tax (Expense) Credit 0.3 0.3 1.0 0.8
Net Current Period Other Comprehensive Income 0.3 0.3 1.0 0.8
Ending Balance in AOCI (15.7) (13.5) (15.7) (13.5)
Indiana Michigan Power Co [Member] | Pension and OPEB [Member]        
Changes in Accumulated Other Comprehensive Income (Loss) by Component        
Beginning Balance in AOCI (3.4) 0.1 (3.4) 0.1
Change in Fair Value Recognized in AOCI 0.0 0.0 0.0 0.0
Reclassifications From Accumulated Other Comprehensive Income [Abstract]        
Reclassifications from AOCI, before Income Tax (Expense) Credit 0.0 0.0 0.0 0.0
Reclassifications from AOCI, Net of Income Tax (Expense) Credit 0.0 0.0 0.0 0.0
Net Current Period Other Comprehensive Income 0.0 0.0 0.0 0.0
Ending Balance in AOCI (3.4) 0.1 (3.4) 0.1
Indiana Michigan Power Co [Member] | Interest Rate and Foreign Currency [Member] | Cash Flow Hedges [Member]        
Changes in Accumulated Other Comprehensive Income (Loss) by Component        
Beginning Balance in AOCI (12.6) (13.9) (13.3) (14.4)
Change in Fair Value Recognized in AOCI 0.0 0.0 0.0 0.0
Reclassifications From Accumulated Other Comprehensive Income [Abstract]        
Reclassifications from AOCI, before Income Tax (Expense) Credit 0.5 0.4 1.5 1.2
Reclassifications from AOCI, Net of Income Tax (Expense) Credit 0.3 0.3 1.0 0.8
Net Current Period Other Comprehensive Income 0.3 0.3 1.0 0.8
Ending Balance in AOCI (12.3) (13.6) (12.3) (13.6)
Indiana Michigan Power Co [Member] | Reclassifications from Accumulated Other Comprehensive Income [Member]        
Reclassifications From Accumulated Other Comprehensive Income [Abstract]        
Interest Expense 0.5 0.4 1.5 1.2
Prior Service Cost (Credit) (0.2) (0.2) (0.6) (0.6)
Actuarial (Gains)/Losses 0.2 0.2 0.6 0.6
Income Tax (Expense) Credit 0.2 0.1 0.5 0.4
Indiana Michigan Power Co [Member] | Reclassifications from Accumulated Other Comprehensive Income [Member] | Pension and OPEB [Member]        
Reclassifications From Accumulated Other Comprehensive Income [Abstract]        
Interest Expense 0.0 0.0 0.0 0.0
Prior Service Cost (Credit) (0.2) (0.2) (0.6) (0.6)
Actuarial (Gains)/Losses 0.2 0.2 0.6 0.6
Income Tax (Expense) Credit 0.0 0.0 0.0 0.0
Indiana Michigan Power Co [Member] | Reclassifications from Accumulated Other Comprehensive Income [Member] | Interest Rate and Foreign Currency [Member] | Cash Flow Hedges [Member]        
Reclassifications From Accumulated Other Comprehensive Income [Abstract]        
Interest Expense 0.5 0.4 1.5 1.2
Prior Service Cost (Credit) 0.0 0.0 0.0 0.0
Actuarial (Gains)/Losses 0.0 0.0 0.0 0.0
Income Tax (Expense) Credit 0.2 0.1 0.5 0.4
Ohio Power Co [Member]        
Changes in Accumulated Other Comprehensive Income (Loss) by Component        
Beginning Balance in AOCI     4.3  
Reclassifications From Accumulated Other Comprehensive Income [Abstract]        
Other Revenues 1.4 2.0 4.8 6.4
Purchased Electricity for Resale 203.4 173.1 516.1 431.6
Interest Expense 27.2 32.6 87.7 96.3
Income Tax (Expense) Credit (46.4) (38.5) (122.5) (100.6)
Net Current Period Other Comprehensive Income     (1.0) (1.0)
Ending Balance in AOCI 3.3   3.3  
Ohio Power Co [Member] | Interest Rate and Foreign Currency [Member] | Cash Flow Hedges [Member]        
Changes in Accumulated Other Comprehensive Income (Loss) by Component        
Beginning Balance in AOCI 3.5 4.9 4.3 5.6
Change in Fair Value Recognized in AOCI 0.0 0.0 0.0 0.0
Reclassifications From Accumulated Other Comprehensive Income [Abstract]        
Reclassifications from AOCI, before Income Tax (Expense) Credit (0.3) (0.5) (1.4) (1.6)
Reclassifications from AOCI, Net of Income Tax (Expense) Credit (0.2) (0.3) (1.0) (1.0)
Net Current Period Other Comprehensive Income (0.2) (0.3) (1.0) (1.0)
Ending Balance in AOCI 3.3 4.6 3.3 4.6
Ohio Power Co [Member] | Reclassifications from Accumulated Other Comprehensive Income [Member] | Interest Rate and Foreign Currency [Member] | Cash Flow Hedges [Member]        
Reclassifications From Accumulated Other Comprehensive Income [Abstract]        
Interest Expense (0.3) (0.5) (1.4) (1.6)
Income Tax (Expense) Credit (0.1) (0.2) (0.4) (0.6)
Public Service Co Of Oklahoma [Member]        
Changes in Accumulated Other Comprehensive Income (Loss) by Component        
Beginning Balance in AOCI     4.2  
Reclassifications From Accumulated Other Comprehensive Income [Abstract]        
Other Revenues 0.7 0.6 2.9 2.2
Purchased Electricity for Resale 130.8 103.2 315.3 253.8
Interest Expense 14.9 15.0 44.6 44.4
Income Tax (Expense) Credit (32.0) (27.4) (56.6) (51.3)
Net Current Period Other Comprehensive Income     (0.6) (0.5)
Ending Balance in AOCI 3.6   3.6  
Public Service Co Of Oklahoma [Member] | Interest Rate and Foreign Currency [Member] | Cash Flow Hedges [Member]        
Changes in Accumulated Other Comprehensive Income (Loss) by Component        
Beginning Balance in AOCI 3.8 4.6 4.2 5.0
Change in Fair Value Recognized in AOCI 0.0 0.0 0.0 0.0
Reclassifications From Accumulated Other Comprehensive Income [Abstract]        
Reclassifications from AOCI, before Income Tax (Expense) Credit (0.3) (0.2) (0.9) (0.8)
Reclassifications from AOCI, Net of Income Tax (Expense) Credit (0.2) (0.1) (0.6) (0.5)
Net Current Period Other Comprehensive Income (0.2) (0.1) (0.6) (0.5)
Ending Balance in AOCI 3.6 4.5 3.6 4.5
Public Service Co Of Oklahoma [Member] | Reclassifications from Accumulated Other Comprehensive Income [Member] | Interest Rate and Foreign Currency [Member] | Cash Flow Hedges [Member]        
Reclassifications From Accumulated Other Comprehensive Income [Abstract]        
Interest Expense (0.3) (0.2) (0.9) (0.8)
Income Tax (Expense) Credit (0.1) (0.1) (0.3) (0.3)
Southwestern Electric Power Co [Member]        
Changes in Accumulated Other Comprehensive Income (Loss) by Component        
Beginning Balance in AOCI (8.9) (6.9) (9.4) (7.5)
Change in Fair Value Recognized in AOCI 0.0 0.0 0.0 0.0
Reclassifications From Accumulated Other Comprehensive Income [Abstract]        
Other Revenues 0.6 0.6 1.6 1.5
Purchased Electricity for Resale 35.9 23.6 97.5 70.8
Interest Expense 32.6 29.2 92.0 91.4
Reclassifications from AOCI, before Income Tax (Expense) Credit 0.5 0.3 1.2 1.3
Income Tax (Expense) Credit (33.2) (37.4) (53.9) (85.4)
Reclassifications from AOCI, Net of Income Tax (Expense) Credit 0.3 0.2 0.8 0.8
Net Current Period Other Comprehensive Income 0.3 0.2 0.8 0.8
Ending Balance in AOCI (8.6) (6.7) (8.6) (6.7)
Southwestern Electric Power Co [Member] | Pension and OPEB [Member]        
Changes in Accumulated Other Comprehensive Income (Loss) by Component        
Beginning Balance in AOCI (0.7) 3.1 (0.3) 3.6
Change in Fair Value Recognized in AOCI 0.0 0.0 0.0 0.0
Reclassifications From Accumulated Other Comprehensive Income [Abstract]        
Reclassifications from AOCI, before Income Tax (Expense) Credit (0.2) (0.4) (0.8) (1.1)
Reclassifications from AOCI, Net of Income Tax (Expense) Credit (0.1) (0.2) (0.5) (0.7)
Net Current Period Other Comprehensive Income (0.1) (0.2) (0.5) (0.7)
Ending Balance in AOCI (0.8) 2.9 (0.8) 2.9
Southwestern Electric Power Co [Member] | Interest Rate and Foreign Currency [Member] | Cash Flow Hedges [Member]        
Changes in Accumulated Other Comprehensive Income (Loss) by Component        
Beginning Balance in AOCI (8.2) (10.0) (9.1) (11.1)
Change in Fair Value Recognized in AOCI 0.0 0.0 0.0 0.0
Reclassifications From Accumulated Other Comprehensive Income [Abstract]        
Reclassifications from AOCI, before Income Tax (Expense) Credit 0.7 0.7 2.0 2.4
Reclassifications from AOCI, Net of Income Tax (Expense) Credit 0.4 0.4 1.3 1.5
Net Current Period Other Comprehensive Income 0.4 0.4 1.3 1.5
Ending Balance in AOCI (7.8) (9.6) (7.8) (9.6)
Southwestern Electric Power Co [Member] | Reclassifications from Accumulated Other Comprehensive Income [Member]        
Reclassifications From Accumulated Other Comprehensive Income [Abstract]        
Interest Expense 0.7 0.7 2.0 2.4
Prior Service Cost (Credit) (0.4) (0.5) (1.4) (1.4)
Actuarial (Gains)/Losses 0.2 0.1 0.6 0.3
Income Tax (Expense) Credit 0.2 0.1 0.4 0.5
Southwestern Electric Power Co [Member] | Reclassifications from Accumulated Other Comprehensive Income [Member] | Pension and OPEB [Member]        
Reclassifications From Accumulated Other Comprehensive Income [Abstract]        
Interest Expense 0.0 0.0 0.0 0.0
Prior Service Cost (Credit) (0.4) (0.5) (1.4) (1.4)
Actuarial (Gains)/Losses 0.2 0.1 0.6 0.3
Income Tax (Expense) Credit (0.1) (0.2) (0.3) (0.4)
Southwestern Electric Power Co [Member] | Reclassifications from Accumulated Other Comprehensive Income [Member] | Interest Rate and Foreign Currency [Member] | Cash Flow Hedges [Member]        
Reclassifications From Accumulated Other Comprehensive Income [Abstract]        
Interest Expense 0.7 0.7 2.0 2.4
Prior Service Cost (Credit) 0.0 0.0 0.0 0.0
Actuarial (Gains)/Losses 0.0 0.0 0.0 0.0
Income Tax (Expense) Credit $ 0.3 $ 0.3 $ 0.7 $ 0.9