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Benefit Plans (Tables) | 9 Months Ended
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Sep. 30, 2025
Defined Benefit Plan Disclosure [Line Items] |
Net Periodic Benefit Costs | The components of net periodic benefit costs for sponsored defined benefit pension and OPEB plans for the three and nine months ended September 30 are included below (in millions). For IPL and WPL, amounts are for their plan participants covered under plans they sponsor, as well as amounts directly assigned to them related to certain participants in the Alliant Energy and Corporate Services sponsored plans.
| Defined Benefit Pension Plans |  | OPEB Plans
| Three Months |  | Nine Months |  | Three Months |  | Nine Months
Alliant Energy | 2025 |  | 2024 |  | 2025 |  | 2024 |  | 2025 |  | 2024 |  | 2025 |  | 2024
Service cost | $1 |  | $1 |  | $3 |  | $3 |  | $— |  | $1 |  | $1 |  | $2
Interest cost | 11 |  | 12 |  | 34 |  | 34 |  | 2 |  | 2 |  | 6 |  | 6
Expected return on plan assets | (13) |  | (14) |  | (40) |  | (41) |  | — |  | (2) |  | (3) |  | (4)
Amortization of prior service credit | (1) |  | — |  | (1) |  | — |  | — |  | — |  | — |  | —
Amortization of actuarial loss | 6 |  | 6 |  | 17 |  | 18 |  | — |  | — |  | — |  | —
| $4 |  | $5 |  | $13 |  | $14 |  | $2 |  | $1 |  | $4 |  | $4
| Defined Benefit Pension Plans |  | OPEB Plans
| Three Months |  | Nine Months |  | Three Months |  | Nine Months
IPL | 2025 |  | 2024 |  | 2025 |  | 2024 |  | 2025 |  | 2024 |  | 2025 |  | 2024
Service cost | $1 |  | $1 |  | $2 |  | $2 |  | $— |  | $— |  | $— |  | $—
Interest cost | 5 |  | 5 |  | 15 |  | 15 |  | — |  | 1 |  | 2 |  | 3
Expected return on plan assets | (7) |  | (7) |  | (19) |  | (20) |  | (1) |  | (1) |  | (3) |  | (3)
Amortization of actuarial loss | 3 |  | 2 |  | 7 |  | 7 |  | — |  | — |  | — |  | —
| $2 |  | $1 |  | $5 |  | $4 |  | ($1) |  | $— |  | ($1) |  | $—
| Defined Benefit Pension Plans |  | OPEB Plans
| Three Months |  | Nine Months |  | Three Months |  | Nine Months
WPL | 2025 |  | 2024 |  | 2025 |  | 2024 |  | 2025 |  | 2024 |  | 2025 |  | 2024
Service cost | $— |  | $— |  | $1 |  | $1 |  | $— |  | $— |  | $— |  | $—
Interest cost | 5 |  | 5 |  | 15 |  | 15 |  | 1 |  | 1 |  | 2 |  | 3
Expected return on plan assets | (5) |  | (6) |  | (17) |  | (18) |  | (1) |  | (1) |  | (1) |  | (1)
Amortization of actuarial loss | 3 |  | 3 |  | 8 |  | 9 |  | — |  | — |  | — |  | —
| $3 |  | $2 |  | $7 |  | $7 |  | $— |  | $— |  | $1 |  | $2
Recognized Compensation Expense And Income Tax Benefits | A summary of compensation expense, including amounts allocated to IPL and WPL, and the related income tax benefits recognized for share-based compensation awards for the three and nine months ended September 30 was as follows (in millions):
| Alliant Energy |  | IPL |  | WPL
| Three Months |  | Nine Months |  | Three Months |  | Nine Months |  | Three Months |  | Nine Months
| 2025 |  | 2024 |  | 2025 |  | 2024 |  | 2025 |  | 2024 |  | 2025 |  | 2024 |  | 2025 |  | 2024 |  | 2025 |  | 2024
Compensation expense | $6 |  | $3 |  | $13 |  | $10 |  | $3 |  | $2 |  | $7 |  | $5 |  | $2 |  | $1 |  | $6 |  | $4
Income tax benefits | 2 |  | 1 |  | 4 |  | 3 |  | 1 |  | — |  | 2 |  | 1 |  | 1 |  | — |  | 2 |  | 1
Schedule Of Equity-based Compensation Plans Activity | For the nine months ended September 30, 2025, performance shares and restricted stock units were granted to key employees under the equity-based compensation plans as follows. These shares and units will be paid out in shares of common stock, and are therefore accounted for as equity awards.
|  |  | Weighted Average
| Grants |  | Grant Date Fair Value
Performance shares (total shareowner return metric) | 105,896 |  | $66.52
Performance shares (net income and environmental metrics) | 118,883 |  | 61.63
Restricted stock units | 100,245 |  | 61.66
IPL [Member] |
Defined Benefit Plan Disclosure [Line Items] |
Net Periodic Benefit Costs | The components of net periodic benefit costs for sponsored defined benefit pension and OPEB plans for the three and nine months ended September 30 are included below (in millions). For IPL and WPL, amounts are for their plan participants covered under plans they sponsor, as well as amounts directly assigned to them related to certain participants in the Alliant Energy and Corporate Services sponsored plans.
| Defined Benefit Pension Plans |  | OPEB Plans
| Three Months |  | Nine Months |  | Three Months |  | Nine Months
Alliant Energy | 2025 |  | 2024 |  | 2025 |  | 2024 |  | 2025 |  | 2024 |  | 2025 |  | 2024
Service cost | $1 |  | $1 |  | $3 |  | $3 |  | $— |  | $1 |  | $1 |  | $2
Interest cost | 11 |  | 12 |  | 34 |  | 34 |  | 2 |  | 2 |  | 6 |  | 6
Expected return on plan assets | (13) |  | (14) |  | (40) |  | (41) |  | — |  | (2) |  | (3) |  | (4)
Amortization of prior service credit | (1) |  | — |  | (1) |  | — |  | — |  | — |  | — |  | —
Amortization of actuarial loss | 6 |  | 6 |  | 17 |  | 18 |  | — |  | — |  | — |  | —
| $4 |  | $5 |  | $13 |  | $14 |  | $2 |  | $1 |  | $4 |  | $4
| Defined Benefit Pension Plans |  | OPEB Plans
| Three Months |  | Nine Months |  | Three Months |  | Nine Months
IPL | 2025 |  | 2024 |  | 2025 |  | 2024 |  | 2025 |  | 2024 |  | 2025 |  | 2024
Service cost | $1 |  | $1 |  | $2 |  | $2 |  | $— |  | $— |  | $— |  | $—
Interest cost | 5 |  | 5 |  | 15 |  | 15 |  | — |  | 1 |  | 2 |  | 3
Expected return on plan assets | (7) |  | (7) |  | (19) |  | (20) |  | (1) |  | (1) |  | (3) |  | (3)
Amortization of actuarial loss | 3 |  | 2 |  | 7 |  | 7 |  | — |  | — |  | — |  | —
| $2 |  | $1 |  | $5 |  | $4 |  | ($1) |  | $— |  | ($1) |  | $—
| Defined Benefit Pension Plans |  | OPEB Plans
| Three Months |  | Nine Months |  | Three Months |  | Nine Months
WPL | 2025 |  | 2024 |  | 2025 |  | 2024 |  | 2025 |  | 2024 |  | 2025 |  | 2024
Service cost | $— |  | $— |  | $1 |  | $1 |  | $— |  | $— |  | $— |  | $—
Interest cost | 5 |  | 5 |  | 15 |  | 15 |  | 1 |  | 1 |  | 2 |  | 3
Expected return on plan assets | (5) |  | (6) |  | (17) |  | (18) |  | (1) |  | (1) |  | (1) |  | (1)
Amortization of actuarial loss | 3 |  | 3 |  | 8 |  | 9 |  | — |  | — |  | — |  | —
| $3 |  | $2 |  | $7 |  | $7 |  | $— |  | $— |  | $1 |  | $2
Recognized Compensation Expense And Income Tax Benefits | A summary of compensation expense, including amounts allocated to IPL and WPL, and the related income tax benefits recognized for share-based compensation awards for the three and nine months ended September 30 was as follows (in millions):
| Alliant Energy |  | IPL |  | WPL
| Three Months |  | Nine Months |  | Three Months |  | Nine Months |  | Three Months |  | Nine Months
| 2025 |  | 2024 |  | 2025 |  | 2024 |  | 2025 |  | 2024 |  | 2025 |  | 2024 |  | 2025 |  | 2024 |  | 2025 |  | 2024
Compensation expense | $6 |  | $3 |  | $13 |  | $10 |  | $3 |  | $2 |  | $7 |  | $5 |  | $2 |  | $1 |  | $6 |  | $4
Income tax benefits | 2 |  | 1 |  | 4 |  | 3 |  | 1 |  | — |  | 2 |  | 1 |  | 1 |  | — |  | 2 |  | 1
Schedule Of Equity-based Compensation Plans Activity | For the nine months ended September 30, 2025, performance shares and restricted stock units were granted to key employees under the equity-based compensation plans as follows. These shares and units will be paid out in shares of common stock, and are therefore accounted for as equity awards.
|  |  | Weighted Average
| Grants |  | Grant Date Fair Value
Performance shares (total shareowner return metric) | 105,896 |  | $66.52
Performance shares (net income and environmental metrics) | 118,883 |  | 61.63
Restricted stock units | 100,245 |  | 61.66
WPL [Member] |
Defined Benefit Plan Disclosure [Line Items] |
Net Periodic Benefit Costs | The components of net periodic benefit costs for sponsored defined benefit pension and OPEB plans for the three and nine months ended September 30 are included below (in millions). For IPL and WPL, amounts are for their plan participants covered under plans they sponsor, as well as amounts directly assigned to them related to certain participants in the Alliant Energy and Corporate Services sponsored plans.
| Defined Benefit Pension Plans |  | OPEB Plans
| Three Months |  | Nine Months |  | Three Months |  | Nine Months
Alliant Energy | 2025 |  | 2024 |  | 2025 |  | 2024 |  | 2025 |  | 2024 |  | 2025 |  | 2024
Service cost | $1 |  | $1 |  | $3 |  | $3 |  | $— |  | $1 |  | $1 |  | $2
Interest cost | 11 |  | 12 |  | 34 |  | 34 |  | 2 |  | 2 |  | 6 |  | 6
Expected return on plan assets | (13) |  | (14) |  | (40) |  | (41) |  | — |  | (2) |  | (3) |  | (4)
Amortization of prior service credit | (1) |  | — |  | (1) |  | — |  | — |  | — |  | — |  | —
Amortization of actuarial loss | 6 |  | 6 |  | 17 |  | 18 |  | — |  | — |  | — |  | —
| $4 |  | $5 |  | $13 |  | $14 |  | $2 |  | $1 |  | $4 |  | $4
| Defined Benefit Pension Plans |  | OPEB Plans
| Three Months |  | Nine Months |  | Three Months |  | Nine Months
IPL | 2025 |  | 2024 |  | 2025 |  | 2024 |  | 2025 |  | 2024 |  | 2025 |  | 2024
Service cost | $1 |  | $1 |  | $2 |  | $2 |  | $— |  | $— |  | $— |  | $—
Interest cost | 5 |  | 5 |  | 15 |  | 15 |  | — |  | 1 |  | 2 |  | 3
Expected return on plan assets | (7) |  | (7) |  | (19) |  | (20) |  | (1) |  | (1) |  | (3) |  | (3)
Amortization of actuarial loss | 3 |  | 2 |  | 7 |  | 7 |  | — |  | — |  | — |  | —
| $2 |  | $1 |  | $5 |  | $4 |  | ($1) |  | $— |  | ($1) |  | $—
| Defined Benefit Pension Plans |  | OPEB Plans
| Three Months |  | Nine Months |  | Three Months |  | Nine Months
WPL | 2025 |  | 2024 |  | 2025 |  | 2024 |  | 2025 |  | 2024 |  | 2025 |  | 2024
Service cost | $— |  | $— |  | $1 |  | $1 |  | $— |  | $— |  | $— |  | $—
Interest cost | 5 |  | 5 |  | 15 |  | 15 |  | 1 |  | 1 |  | 2 |  | 3
Expected return on plan assets | (5) |  | (6) |  | (17) |  | (18) |  | (1) |  | (1) |  | (1) |  | (1)
Amortization of actuarial loss | 3 |  | 3 |  | 8 |  | 9 |  | — |  | — |  | — |  | —
| $3 |  | $2 |  | $7 |  | $7 |  | $— |  | $— |  | $1 |  | $2
Recognized Compensation Expense And Income Tax Benefits | A summary of compensation expense, including amounts allocated to IPL and WPL, and the related income tax benefits recognized for share-based compensation awards for the three and nine months ended September 30 was as follows (in millions):
| Alliant Energy |  | IPL |  | WPL
| Three Months |  | Nine Months |  | Three Months |  | Nine Months |  | Three Months |  | Nine Months
| 2025 |  | 2024 |  | 2025 |  | 2024 |  | 2025 |  | 2024 |  | 2025 |  | 2024 |  | 2025 |  | 2024 |  | 2025 |  | 2024
Compensation expense | $6 |  | $3 |  | $13 |  | $10 |  | $3 |  | $2 |  | $7 |  | $5 |  | $2 |  | $1 |  | $6 |  | $4
Income tax benefits | 2 |  | 1 |  | 4 |  | 3 |  | 1 |  | — |  | 2 |  | 1 |  | 1 |  | — |  | 2 |  | 1
Schedule Of Equity-based Compensation Plans Activity | For the nine months ended September 30, 2025, performance shares and restricted stock units were granted to key employees under the equity-based compensation plans as follows. These shares and units will be paid out in shares of common stock, and are therefore accounted for as equity awards.
|  |  | Weighted Average
| Grants |  | Grant Date Fair Value
Performance shares (total shareowner return metric) | 105,896 |  | $66.52
Performance shares (net income and environmental metrics) | 118,883 |  | 61.63
Restricted stock units | 100,245 |  | 61.66

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