Benefit Plans (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Pension Plans [Member]
Components of Net Periodic Benefit Cost
Service Cost	$ 24.1	$ 21.5	$ 48.2	$ 42.9
Interest Cost	50.8	52.9	101.6	105.8
Expected Return on Plan Assets	(71.2)	(70.0)	(142.4)	(140.1)
Amortization of Prior Service Cost (Credit)	0.2	0.5	0.5	1.1
Amortization of Net Actuarial Loss	20.7	20.9	41.4	41.9
Net Periodic Benefit Cost (Credit)	24.6	25.8	49.3	51.6
Pension Plans [Member] | Appalachian Power Co [Member]
Components of Net Periodic Benefit Cost
Service Cost	2.4	2.0	4.7	4.0
Interest Cost	6.4	6.8	12.8	13.6
Expected Return on Plan Assets	(9.0)	(8.9)	(17.9)	(17.7)
Amortization of Prior Service Cost (Credit)	0.0	0.1	0.1	0.1
Amortization of Net Actuarial Loss	2.6	2.7	5.2	5.4
Net Periodic Benefit Cost (Credit)	2.4	2.7	4.9	5.4
Pension Plans [Member] | Indiana Michigan Power Co [Member]
Components of Net Periodic Benefit Cost
Service Cost	3.5	3.1	7.0	6.1
Interest Cost	6.0	6.4	12.1	12.7
Expected Return on Plan Assets	(8.7)	(8.4)	(17.3)	(16.8)
Amortization of Prior Service Cost (Credit)	0.1	0.0	0.1	0.1
Amortization of Net Actuarial Loss	2.5	2.4	4.9	4.9
Net Periodic Benefit Cost (Credit)	3.4	3.5	6.8	7.0
Pension Plans [Member] | Ohio Power Co [Member]
Components of Net Periodic Benefit Cost
Service Cost	1.9	1.7	3.8	3.3
Interest Cost	4.9	5.1	9.7	10.3
Expected Return on Plan Assets	(7.0)	(7.0)	(14.0)	(13.9)
Amortization of Prior Service Cost (Credit)	0.1	0.1	0.1	0.1
Amortization of Net Actuarial Loss	1.9	2.0	3.9	4.0
Net Periodic Benefit Cost (Credit)	1.8	1.9	3.5	3.8
Pension Plans [Member] | Public Service Co Of Oklahoma [Member]
Components of Net Periodic Benefit Cost
Service Cost	1.6	1.6	3.2	3.1
Interest Cost	2.7	2.8	5.4	5.6
Expected Return on Plan Assets	(4.0)	(3.8)	(7.9)	(7.7)
Amortization of Prior Service Cost (Credit)	0.0	0.0	0.0	0.1
Amortization of Net Actuarial Loss	1.1	1.1	2.2	2.2
Net Periodic Benefit Cost (Credit)	1.4	1.7	2.9	3.3
Pension Plans [Member] | Southwestern Electric Power Co [Member]
Components of Net Periodic Benefit Cost
Service Cost	2.2	2.1	4.4	4.1
Interest Cost	3.0	3.1	6.1	6.2
Expected Return on Plan Assets	(4.2)	(4.2)	(8.4)	(8.3)
Amortization of Prior Service Cost (Credit)	0.0	0.1	0.0	0.2
Amortization of Net Actuarial Loss	1.2	1.2	2.4	2.4
Net Periodic Benefit Cost (Credit)	2.2	2.3	4.5	4.6
Other Postretirement Benefit Plans [Member]
Components of Net Periodic Benefit Cost
Service Cost	2.8	2.5	5.6	5.1
Interest Cost	14.9	15.2	29.7	30.4
Expected Return on Plan Assets	(25.4)	(26.7)	(50.7)	(53.5)
Amortization of Prior Service Cost (Credit)	(17.2)	(17.2)	(34.5)	(34.5)
Amortization of Net Actuarial Loss	9.1	7.8	18.3	15.7
Net Periodic Benefit Cost (Credit)	(15.8)	(18.4)	(31.6)	(36.8)
Other Postretirement Benefit Plans [Member] | Appalachian Power Co [Member]
Components of Net Periodic Benefit Cost
Service Cost	0.2	0.3	0.5	0.5
Interest Cost	2.7	2.7	5.3	5.4
Expected Return on Plan Assets	(4.1)	(4.4)	(8.2)	(8.7)
Amortization of Prior Service Cost (Credit)	(2.5)	(2.5)	(5.0)	(5.0)
Amortization of Net Actuarial Loss	1.5	1.3	3.1	2.7
Net Periodic Benefit Cost (Credit)	(2.2)	(2.6)	(4.3)	(5.1)
Other Postretirement Benefit Plans [Member] | Indiana Michigan Power Co [Member]
Components of Net Periodic Benefit Cost
Service Cost	0.4	0.3	0.8	0.7
Interest Cost	1.8	1.7	3.5	3.5
Expected Return on Plan Assets	(3.0)	(3.2)	(6.1)	(6.4)
Amortization of Prior Service Cost (Credit)	(2.4)	(2.3)	(4.7)	(4.7)
Amortization of Net Actuarial Loss	1.1	1.0	2.2	1.9
Net Periodic Benefit Cost (Credit)	(2.1)	(2.5)	(4.3)	(5.0)
Other Postretirement Benefit Plans [Member] | Ohio Power Co [Member]
Components of Net Periodic Benefit Cost
Service Cost	0.2	0.2	0.4	0.4
Interest Cost	1.7	1.8	3.4	3.5
Expected Return on Plan Assets	(3.0)	(3.2)	(6.0)	(6.4)
Amortization of Prior Service Cost (Credit)	(1.8)	(1.8)	(3.5)	(3.5)
Amortization of Net Actuarial Loss	1.1	1.0	2.2	1.9
Net Periodic Benefit Cost (Credit)	(1.8)	(2.0)	(3.5)	(4.1)
Other Postretirement Benefit Plans [Member] | Public Service Co Of Oklahoma [Member]
Components of Net Periodic Benefit Cost
Service Cost	0.1	0.1	0.3	0.3
Interest Cost	0.8	0.8	1.6	1.6
Expected Return on Plan Assets	(1.4)	(1.5)	(2.8)	(3.0)
Amortization of Prior Service Cost (Credit)	(1.0)	(1.0)	(2.1)	(2.1)
Amortization of Net Actuarial Loss	0.5	0.5	1.0	0.9
Net Periodic Benefit Cost (Credit)	(1.0)	(1.1)	(2.0)	(2.3)
Other Postretirement Benefit Plans [Member] | Southwestern Electric Power Co [Member]
Components of Net Periodic Benefit Cost
Service Cost	0.2	0.2	0.4	0.4
Interest Cost	0.9	0.9	1.8	1.8
Expected Return on Plan Assets	(1.6)	(1.6)	(3.2)	(3.3)
Amortization of Prior Service Cost (Credit)	(1.3)	(1.3)	(2.6)	(2.6)
Amortization of Net Actuarial Loss	0.6	0.5	1.2	1.0
Net Periodic Benefit Cost (Credit)	$ (1.2)	$ (1.3)	$ (2.4)	$ (2.7)