Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Changes in Accumulated Other Comprehensive Income (Loss) by Component
Beginning Balance in AOCI	$ (156.3)	$ (127.1)
Change in Fair Value Recognized in AOCI	(20.6)	(7.5)
Reclassifications From Accumulated Other Comprehensive Income [Abstract]
Generation & Marketing Revenues	38.3	35.6
Purchased Electricity for Resale	769.6	731.4
Interest Expense	221.8	217.0
Reclassifications from AOCI, before Income Tax (Expense) Credit	9.0	1.3
Income Tax (Expense) Credit	(343.2)	(235.5)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	5.9	0.8
Net Current Period Other Comprehensive Income	(14.7)	(6.7)
Ending Balance in AOCI	(171.0)	(133.8)
Securities Available for Sale [Member]
Changes in Accumulated Other Comprehensive Income (Loss) by Component
Beginning Balance in AOCI	8.4	7.1
Change in Fair Value Recognized in AOCI	1.2	0.6
Reclassifications From Accumulated Other Comprehensive Income [Abstract]
Reclassifications from AOCI, before Income Tax (Expense) Credit	0.0	0.0
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	0.0	0.0
Net Current Period Other Comprehensive Income	1.2	0.6
Ending Balance in AOCI	9.6	7.7
Pension and OPEB [Member]
Changes in Accumulated Other Comprehensive Income (Loss) by Component
Beginning Balance in AOCI	(125.9)	(111.8)
Change in Fair Value Recognized in AOCI	0.0	0.0
Reclassifications From Accumulated Other Comprehensive Income [Abstract]
Reclassifications from AOCI, before Income Tax (Expense) Credit	0.4	0.2
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	0.2	0.1
Net Current Period Other Comprehensive Income	0.2	0.1
Ending Balance in AOCI	(125.7)	(111.7)
Commodity [Member] | Cash Flow Hedges [Member]
Changes in Accumulated Other Comprehensive Income (Loss) by Component
Beginning Balance in AOCI	(23.1)	(5.2)
Change in Fair Value Recognized in AOCI	(21.8)	(8.1)
Reclassifications From Accumulated Other Comprehensive Income [Abstract]
Reclassifications from AOCI, before Income Tax (Expense) Credit	8.1	0.6
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	5.3	0.4
Net Current Period Other Comprehensive Income	(16.5)	(7.7)
Ending Balance in AOCI	(39.6)	(12.9)
Interest Rate and Foreign Currency [Member] | Cash Flow Hedges [Member]
Changes in Accumulated Other Comprehensive Income (Loss) by Component
Beginning Balance in AOCI	(15.7)	(17.2)
Change in Fair Value Recognized in AOCI	0.0	0.0
Reclassifications From Accumulated Other Comprehensive Income [Abstract]
Reclassifications from AOCI, before Income Tax (Expense) Credit	0.5	0.5
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	0.4	0.3
Net Current Period Other Comprehensive Income	0.4	0.3
Ending Balance in AOCI	(15.3)	(16.9)
Reclassifications from Accumulated Other Comprehensive Income [Member]
Reclassifications From Accumulated Other Comprehensive Income [Abstract]
Generation & Marketing Revenues	(4.7)	(8.6)
Purchased Electricity for Resale	12.8	9.2
Interest Expense	0.5	0.5
Amortization of Prior Service Cost (Credit)	(4.9)	(4.9)
Amortization of Actuarial (Gains)/Losses	5.3	5.1
Income Tax (Expense) Credit	3.1	0.5
Reclassifications from Accumulated Other Comprehensive Income [Member] | Securities Available for Sale [Member]
Reclassifications From Accumulated Other Comprehensive Income [Abstract]
Generation & Marketing Revenues	0.0	0.0
Purchased Electricity for Resale	0.0	0.0
Interest Expense	0.0	0.0
Amortization of Prior Service Cost (Credit)	0.0	0.0
Amortization of Actuarial (Gains)/Losses	0.0	0.0
Income Tax (Expense) Credit	0.0	0.0
Reclassifications from Accumulated Other Comprehensive Income [Member] | Pension and OPEB [Member]
Reclassifications From Accumulated Other Comprehensive Income [Abstract]
Generation & Marketing Revenues	0.0	0.0
Purchased Electricity for Resale	0.0	0.0
Interest Expense	0.0	0.0
Amortization of Prior Service Cost (Credit)	(4.9)	(4.9)
Amortization of Actuarial (Gains)/Losses	5.3	5.1
Income Tax (Expense) Credit	0.2	0.1
Reclassifications from Accumulated Other Comprehensive Income [Member] | Commodity [Member] | Cash Flow Hedges [Member]
Reclassifications From Accumulated Other Comprehensive Income [Abstract]
Generation & Marketing Revenues	(4.7)	(8.6)
Purchased Electricity for Resale	12.8	9.2
Interest Expense	0.0	0.0
Amortization of Prior Service Cost (Credit)	0.0	0.0
Amortization of Actuarial (Gains)/Losses	0.0	0.0
Income Tax (Expense) Credit	2.8	0.2
Reclassifications from Accumulated Other Comprehensive Income [Member] | Interest Rate and Foreign Currency [Member] | Cash Flow Hedges [Member]
Reclassifications From Accumulated Other Comprehensive Income [Abstract]
Generation & Marketing Revenues	0.0	0.0
Purchased Electricity for Resale	0.0	0.0
Interest Expense	0.5	0.5
Amortization of Prior Service Cost (Credit)	0.0	0.0
Amortization of Actuarial (Gains)/Losses	0.0	0.0
Income Tax (Expense) Credit	0.1	0.2
Appalachian Power Co [Member]
Changes in Accumulated Other Comprehensive Income (Loss) by Component
Beginning Balance in AOCI	(8.4)	(2.8)
Change in Fair Value Recognized in AOCI	0.0	0.0
Reclassifications From Accumulated Other Comprehensive Income [Abstract]
Generation & Marketing Revenues	5.4	4.1
Purchased Electricity for Resale	90.8	108.2
Interest Expense	48.1	47.0
Reclassifications from AOCI, before Income Tax (Expense) Credit	(0.8)	(0.8)
Income Tax (Expense) Credit	(63.6)	(72.9)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	(0.5)	(0.5)
Net Current Period Other Comprehensive Income	(0.5)	(0.5)
Ending Balance in AOCI	(8.9)	(3.3)
Appalachian Power Co [Member] | Pension and OPEB [Member]
Changes in Accumulated Other Comprehensive Income (Loss) by Component
Beginning Balance in AOCI	(11.3)	(6.4)
Change in Fair Value Recognized in AOCI	0.0	0.0
Reclassifications From Accumulated Other Comprehensive Income [Abstract]
Reclassifications from AOCI, before Income Tax (Expense) Credit	(0.5)	(0.5)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	(0.3)	(0.3)
Net Current Period Other Comprehensive Income	(0.3)	(0.3)
Ending Balance in AOCI	(11.6)	(6.7)
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	2.9	3.6
Change in Fair Value Recognized in AOCI	0.0	0.0
Reclassifications From Accumulated Other Comprehensive Income [Abstract]
Reclassifications from AOCI, before Income Tax (Expense) Credit	(0.3)	(0.3)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	(0.2)	(0.2)
Net Current Period Other Comprehensive Income	(0.2)	(0.2)
Ending Balance in AOCI	2.7	3.4
Appalachian Power Co [Member] | Reclassifications from Accumulated Other Comprehensive Income [Member]
Reclassifications From Accumulated Other Comprehensive Income [Abstract]
Interest Expense	(0.3)	(0.3)
Amortization of Prior Service Cost (Credit)	(1.3)	(1.2)
Amortization of Actuarial (Gains)/Losses	0.8	0.7
Income Tax (Expense) Credit	(0.3)	(0.3)
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
Amortization of Prior Service Cost (Credit)	(1.3)	(1.2)
Amortization of Actuarial (Gains)/Losses	0.8	0.7
Income Tax (Expense) Credit	(0.2)	(0.2)
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	(0.3)	(0.3)
Amortization of Prior Service Cost (Credit)	0.0	0.0
Amortization of Actuarial (Gains)/Losses	0.0	0.0
Income Tax (Expense) Credit	(0.1)	(0.1)
Indiana Michigan Power Co [Member]
Changes in Accumulated Other Comprehensive Income (Loss) by Component
Beginning Balance in AOCI	(16.2)	(16.7)
Change in Fair Value Recognized in AOCI	0.0	0.0
Reclassifications From Accumulated Other Comprehensive Income [Abstract]
Generation & Marketing Revenues	3.3	5.5
Purchased Electricity for Resale	37.3	49.6
Interest Expense	27.7	22.5
Reclassifications from AOCI, before Income Tax (Expense) Credit	0.5	0.5
Income Tax (Expense) Credit	(29.2)	(24.1)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	0.3	0.4
Net Current Period Other Comprehensive Income	0.3	0.4
Ending Balance in AOCI	(15.9)	(16.3)
Indiana Michigan Power Co [Member] | Pension and OPEB [Member]
Changes in Accumulated Other Comprehensive Income (Loss) by Component
Beginning Balance in AOCI	(4.2)	(3.4)
Change in Fair Value Recognized in AOCI	0.0	0.0
Reclassifications From Accumulated Other Comprehensive Income [Abstract]
Reclassifications from AOCI, before Income Tax (Expense) Credit	0.0	0.0
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	0.0	0.0
Net Current Period Other Comprehensive Income	0.0	0.0
Ending Balance in AOCI	(4.2)	(3.4)
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	(12.0)	(13.3)
Change in Fair Value Recognized in AOCI	0.0	0.0
Reclassifications From Accumulated Other Comprehensive Income [Abstract]
Reclassifications from AOCI, before Income Tax (Expense) Credit	0.5	0.5
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	0.3	0.4
Net Current Period Other Comprehensive Income	0.3	0.4
Ending Balance in AOCI	(11.7)	(12.9)
Indiana Michigan Power Co [Member] | Reclassifications from Accumulated Other Comprehensive Income [Member]
Reclassifications From Accumulated Other Comprehensive Income [Abstract]
Interest Expense	0.5	0.5
Amortization of Prior Service Cost (Credit)	(0.2)	(0.2)
Amortization of Actuarial (Gains)/Losses	0.2	0.2
Income Tax (Expense) Credit	0.2	0.1
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
Amortization of Prior Service Cost (Credit)	(0.2)	(0.2)
Amortization of Actuarial (Gains)/Losses	0.2	0.2
Income Tax (Expense) Credit	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	0.5
Amortization of Prior Service Cost (Credit)	0.0	0.0
Amortization of Actuarial (Gains)/Losses	0.0	0.0
Income Tax (Expense) Credit	0.2	0.1
Ohio Power Co [Member]
Changes in Accumulated Other Comprehensive Income (Loss) by Component
Beginning Balance in AOCI	3.0
Reclassifications From Accumulated Other Comprehensive Income [Abstract]
Generation & Marketing Revenues	2.0	2.1
Purchased Electricity for Resale	188.3	164.9
Interest Expense	25.0	31.4
Income Tax (Expense) Credit	(46.3)	(37.5)
Net Current Period Other Comprehensive Income	(0.2)	(0.4)
Ending Balance in AOCI	2.8
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	3.0	4.3
Change in Fair Value Recognized in AOCI	0.0	0.0
Reclassifications From Accumulated Other Comprehensive Income [Abstract]
Reclassifications from AOCI, before Income Tax (Expense) Credit	(0.4)	(0.5)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	(0.2)	(0.4)
Net Current Period Other Comprehensive Income	(0.2)	(0.4)
Ending Balance in AOCI	2.8	3.9
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	(0.4)	(0.5)
Income Tax (Expense) Credit	(0.2)	(0.1)
Public Service Co Of Oklahoma [Member]
Changes in Accumulated Other Comprehensive Income (Loss) by Component
Beginning Balance in AOCI	3.4
Reclassifications From Accumulated Other Comprehensive Income [Abstract]
Generation & Marketing Revenues	1.1	1.5
Purchased Electricity for Resale	125.3	93.3
Interest Expense	13.6	14.4
Income Tax (Expense) Credit	(2.9)	(8.2)
Net Current Period Other Comprehensive Income	(0.2)	(0.2)
Ending Balance in AOCI	3.2
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	3.4	4.2
Change in Fair Value Recognized in AOCI	0.0	0.0
Reclassifications From Accumulated Other Comprehensive Income [Abstract]
Reclassifications from AOCI, before Income Tax (Expense) Credit	(0.3)	(0.3)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	(0.2)	(0.2)
Net Current Period Other Comprehensive Income	(0.2)	(0.2)
Ending Balance in AOCI	3.2	4.0
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)	(0.3)
Income Tax (Expense) Credit	(0.1)	(0.1)
Southwestern Electric Power Co [Member]
Changes in Accumulated Other Comprehensive Income (Loss) by Component
Beginning Balance in AOCI	(9.4)	(9.4)
Change in Fair Value Recognized in AOCI	0.0	0.0
Reclassifications From Accumulated Other Comprehensive Income [Abstract]
Generation & Marketing Revenues	0.4	0.5
Purchased Electricity for Resale	32.4	28.1
Interest Expense	29.9	27.9
Reclassifications from AOCI, before Income Tax (Expense) Credit	0.4	0.4
Income Tax (Expense) Credit	(9.5)	(7.4)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	0.3	0.3
Net Current Period Other Comprehensive Income	0.3	0.3
Ending Balance in AOCI	(9.1)	(9.1)
Southwestern Electric Power Co [Member] | Pension and OPEB [Member]
Changes in Accumulated Other Comprehensive Income (Loss) by Component
Beginning Balance in AOCI	(2.0)	(0.3)
Change in Fair Value Recognized in AOCI	0.0	0.0
Reclassifications From Accumulated Other Comprehensive Income [Abstract]
Reclassifications from AOCI, before Income Tax (Expense) Credit	(0.3)	(0.3)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	(0.2)	(0.2)
Net Current Period Other Comprehensive Income	(0.2)	(0.2)
Ending Balance in AOCI	(2.2)	(0.5)
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	(7.4)	(9.1)
Change in Fair Value Recognized in AOCI	0.0	0.0
Reclassifications From Accumulated Other Comprehensive Income [Abstract]
Reclassifications from AOCI, before Income Tax (Expense) Credit	0.7	0.7
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	0.5	0.5
Net Current Period Other Comprehensive Income	0.5	0.5
Ending Balance in AOCI	(6.9)	(8.6)
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
Amortization of Prior Service Cost (Credit)	(0.5)	(0.5)
Amortization of Actuarial (Gains)/Losses	0.2	0.2
Income Tax (Expense) Credit	0.1	0.1
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
Amortization of Prior Service Cost (Credit)	(0.5)	(0.5)
Amortization of Actuarial (Gains)/Losses	0.2	0.2
Income Tax (Expense) Credit	(0.1)	(0.1)
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
Amortization of Prior Service Cost (Credit)	0.0	0.0
Amortization of Actuarial (Gains)/Losses	0.0	0.0
Income Tax (Expense) Credit	$ 0.2	$ 0.2