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BENEFIT PLANS - Components of Net Periodic Benefit Cost (Credit) (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2025 | Sep. 30, 2024 | Sep. 30, 2025 | Sep. 30, 2024
Pension Plans |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Service Cost | $ 23.9 | $ 25.6 | $ 71.8 | $ 76.8
Interest Cost | 52.9 | 51.9 | 158.5 | 155.7
Expected Return on Plan Assets | (70.3) | (80.2) | (210.8) | (240.6)
Amortization of Net Actuarial Gain (Loss) | 4.1 | 1.0 | 12.2 | 3.2
Net Periodic Benefit Cost (Credit) | 10.6 | (1.7) | 31.7 | (4.9)
Pension Plans | AEP Texas |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Service Cost | 2.2 | 2.3 | 6.6 | 6.7
Interest Cost | 4.4 | 4.4 | 13.2 | 13.1
Expected Return on Plan Assets | (5.6) | (6.5) | (16.7) | (19.5)
Amortization of Net Actuarial Gain (Loss) | 0.3 | 0.0 | 1.0 | 0.2
Net Periodic Benefit Cost (Credit) | 1.3 | 0.2 | 4.1 | 0.5
Pension Plans | APCo |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Service Cost | 2.3 | 2.4 | 6.9 | 7.3
Interest Cost | 6.2 | 6.2 | 18.7 | 18.6
Expected Return on Plan Assets | (9.4) | (10.7) | (28.2) | (32.1)
Amortization of Net Actuarial Gain (Loss) | 0.5 | 0.1 | 1.4 | 0.3
Net Periodic Benefit Cost (Credit) | (0.4) | (2.0) | (1.2) | (5.9)
Pension Plans | I&M |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Service Cost | 3.2 | 3.3 | 9.5 | 9.9
Interest Cost | 6.2 | 5.9 | 18.6 | 17.8
Expected Return on Plan Assets | (9.7) | (10.7) | (29.1) | (32.1)
Amortization of Net Actuarial Gain (Loss) | 0.4 | 0.1 | 1.3 | 0.3
Net Periodic Benefit Cost (Credit) | 0.1 | (1.4) | 0.3 | (4.1)
Pension Plans | OPCo |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Service Cost | 2.2 | 2.3 | 6.8 | 7.0
Interest Cost | 4.9 | 4.8 | 14.6 | 14.2
Expected Return on Plan Assets | (7.3) | (8.2) | (22.0) | (24.5)
Amortization of Net Actuarial Gain (Loss) | 0.4 | 0.1 | 1.1 | 0.2
Net Periodic Benefit Cost (Credit) | 0.2 | (1.0) | 0.5 | (3.1)
Pension Plans | PSO |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Service Cost | 1.5 | 1.5 | 4.5 | 4.6
Interest Cost | 2.5 | 2.5 | 7.7 | 7.5
Expected Return on Plan Assets | (3.8) | (4.3) | (11.5) | (13.0)
Amortization of Net Actuarial Gain (Loss) | 0.3 | 0.0 | 0.6 | 0.1
Net Periodic Benefit Cost (Credit) | 0.5 | (0.3) | 1.3 | (0.8)
Pension Plans | SWEPCo |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Service Cost | 1.9 | 1.9 | 5.7 | 5.9
Interest Cost | 3.2 | 3.1 | 9.4 | 9.3
Expected Return on Plan Assets | (3.7) | (4.4) | (11.0) | (13.2)
Amortization of Net Actuarial Gain (Loss) | 0.2 | 0.1 | 0.7 | 0.2
Net Periodic Benefit Cost (Credit) | 1.6 | 0.7 | 4.8 | 2.2
OPEB |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Service Cost | 0.9 | 1.1 | 2.7 | 3.3
Interest Cost | 8.5 | 10.3 | 25.7 | 31.4
Expected Return on Plan Assets | (28.2) | (27.8) | (84.7) | (83.5)
Amortization of Prior Service Credit | (0.6) | (3.1) | (1.8) | (9.5)
Amortization of Net Actuarial Gain (Loss) | (0.3) | 0.8 | (0.9) | 2.3
Net Periodic Benefit Cost (Credit) | (19.7) | (18.7) | (59.0) | (56.0)
OPEB | AEP Texas |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Service Cost | 0.0 | 0.0 | 0.2 | 0.2
Interest Cost | 0.7 | 0.9 | 2.0 | 2.5
Expected Return on Plan Assets | (2.3) | (2.3) | (7.0) | (6.8)
Amortization of Prior Service Credit | 0.0 | (0.3) | (0.1) | (0.8)
Amortization of Net Actuarial Gain (Loss) | 0.0 | 0.1 | 0.0 | 0.2
Net Periodic Benefit Cost (Credit) | (1.6) | (1.6) | (4.9) | (4.7)
OPEB | APCo |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Service Cost | 0.0 | 0.2 | 0.3 | 0.4
Interest Cost | 1.4 | 1.7 | 4.1 | 5.0
Expected Return on Plan Assets | (4.1) | (4.1) | (12.3) | (12.2)
Amortization of Prior Service Credit | (0.1) | (0.5) | (0.3) | (1.4)
Amortization of Net Actuarial Gain (Loss) | (0.1) | 0.1 | (0.3) | 0.3
Net Periodic Benefit Cost (Credit) | (2.9) | (2.6) | (8.5) | (7.9)
OPEB | I&M |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Service Cost | 0.0 | 0.1 | 0.3 | 0.4
Interest Cost | 1.0 | 1.2 | 2.9 | 3.6
Expected Return on Plan Assets | (3.4) | (3.4) | (10.1) | (10.1)
Amortization of Prior Service Credit | 0.0 | (0.4) | (0.2) | (1.3)
Amortization of Net Actuarial Gain (Loss) | 0.0 | 0.1 | 0.0 | 0.3
Net Periodic Benefit Cost (Credit) | (2.4) | (2.4) | (7.1) | (7.1)
OPEB | OPCo |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Service Cost | 0.1 | 0.0 | 0.3 | 0.2
Interest Cost | 0.8 | 1.1 | 2.5 | 3.2
Expected Return on Plan Assets | (2.9) | (3.0) | (8.8) | (8.9)
Amortization of Prior Service Credit | (0.1) | (0.3) | (0.2) | (0.9)
Amortization of Net Actuarial Gain (Loss) | 0.0 | 0.1 | 0.0 | 0.3
Net Periodic Benefit Cost (Credit) | (2.1) | (2.1) | (6.2) | (6.1)
OPEB | PSO |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Service Cost | 0.1 | 0.1 | 0.2 | 0.2
Interest Cost | 0.4 | 0.6 | 1.3 | 1.7
Expected Return on Plan Assets | (1.6) | (1.5) | (4.6) | (4.5)
Amortization of Prior Service Credit | 0.0 | (0.2) | (0.1) | (0.6)
Amortization of Net Actuarial Gain (Loss) | 0.0 | 0.0 | 0.0 | 0.1
Net Periodic Benefit Cost (Credit) | (1.1) | (1.0) | (3.2) | (3.1)
OPEB | SWEPCo |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Service Cost | 0.1 | 0.1 | 0.2 | 0.3
Interest Cost | 0.5 | 0.7 | 1.6 | 2.0
Expected Return on Plan Assets | (1.9) | (1.9) | (5.7) | (5.6)
Amortization of Prior Service Credit | 0.0 | (0.3) | (0.1) | (0.8)
Amortization of Net Actuarial Gain (Loss) | 0.0 | 0.1 | 0.0 | 0.2
Net Periodic Benefit Cost (Credit) | $ (1.3) | $ (1.3) | $ (4.0) | $ (3.9)

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