XML 51 R28.htm IDEA: XBRL DOCUMENT
v3.8.0.1

Benefit Plans (Tables) | 9 Months Ended
--- | ---
Sep. 30, 2017
Defined Benefit Plan Disclosure [Line Items] |
Defined Benefit Pension And Other Postretirement Benefits Plans | 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 in the tables below (in millions). In IPL’s and WPL’s tables below, the defined benefit pension plan amounts represent those respective amounts for their bargaining unit employees covered under the qualified plans that they sponsor, as well as amounts directly assigned to them related to their current and former non-bargaining employees who are participants in the Alliant Energy and Corporate Services sponsored qualified and non-qualified defined benefit pension plans. In IPL’s and WPL’s tables below, the OPEB plans amounts represent respective amounts for their employees, as well as amounts directly assigned to them related to their current and former non-bargaining employees who are participants in the Corporate Services sponsored OPEB plan.
| Defined Benefit Pension Plans |  | OPEB Plans
| Three Months |  | Nine Months |  | Three Months |  | Nine Months
Alliant Energy | 2017 |  | 2016 |  | 2017 |  | 2016 |  | 2017 |  | 2016 |  | 2017 |  | 2016
Service cost |  | $3.1 |  |  |  | $3.2 |  |  |  | $9.3 |  |  |  | $9.5 |  |  |  | $1.2 |  |  |  | $1.4 |  |  |  | $3.7 |  |  |  | $4.0 |
Interest cost | 12.7 |  |  | 13.2 |  |  | 38.3 |  |  | 39.7 |  |  | 2.2 |  |  | 2.3 |  |  | 6.5 |  |  | 7.0 |
Expected return on plan assets | (16.3 | ) |  | (16.3 | ) |  | (49.1 | ) |  | (49.1 | ) |  | (1.5 | ) |  | (1.6 | ) |  | (4.6 | ) |  | (4.6 | )
Amortization of prior service credit | (0.1 | ) |  | (0.1 | ) |  | (0.3 | ) |  | (0.2 | ) |  | (0.1 | ) |  | (1.0 | ) |  | (0.2 | ) |  | (3.1 | )
Amortization of actuarial loss | 9.4 |  |  | 9.3 |  |  | 28.2 |  |  | 28.0 |  |  | 1.0 |  |  | 1.2 |  |  | 2.9 |  |  | 3.6 |
Settlement losses (a) | 0.9 |  |  | — |  |  | 0.9 |  |  | — |  |  | — |  |  | — |  |  | — |  |  | — |
|  | $9.7 |  |  |  | $9.3 |  |  |  | $27.3 |  |  |  | $27.9 |  |  |  | $2.8 |  |  |  | $2.3 |  |  |  | $8.3 |  |  |  | $6.9 |
| Defined Benefit Pension Plans |  | OPEB Plans
| Three Months |  | Nine Months |  | Three Months |  | Nine Months
IPL | 2017 |  | 2016 |  | 2017 |  | 2016 |  | 2017 |  | 2016 |  | 2017 |  | 2016
Service cost |  | $1.8 |  |  |  | $1.8 |  |  |  | $5.5 |  |  |  | $5.6 |  |  |  | $0.5 |  |  |  | $0.5 |  |  |  | $1.6 |  |  |  | $1.7 |
Interest cost | 5.9 |  |  | 6.1 |  |  | 17.6 |  |  | 18.4 |  |  | 0.8 |  |  | 1.0 |  |  | 2.6 |  |  | 2.9 |
Expected return on plan assets | (7.7 | ) |  | (7.7 | ) |  | (23.1 | ) |  | (23.2 | ) |  | (1.0 | ) |  | (1.0 | ) |  | (3.2 | ) |  | (3.2 | )
Amortization of prior service credit | — |  |  | — |  |  | (0.1 | ) |  | (0.1 | ) |  | — |  |  | (0.7 | ) |  | — |  |  | (2.0 | )
Amortization of actuarial loss | 4.0 |  |  | 4.2 |  |  | 12.1 |  |  | 12.4 |  |  | 0.5 |  |  | 0.7 |  |  | 1.5 |  |  | 2.0 |
|  | $4.0 |  |  |  | $4.4 |  |  |  | $12.0 |  |  |  | $13.1 |  |  |  | $0.8 |  |  |  | $0.5 |  |  |  | $2.5 |  |  |  | $1.4 |
| Defined Benefit Pension Plans |  | OPEB Plans
| Three Months |  | Nine Months |  | Three Months |  | Nine Months
WPL | 2017 |  | 2016 |  | 2017 |  | 2016 |  | 2017 |  | 2016 |  | 2017 |  | 2016
Service cost |  | $1.2 |  |  |  | $1.3 |  |  |  | $3.6 |  |  |  | $3.7 |  |  |  | $0.5 |  |  |  | $0.5 |  |  |  | $1.4 |  |  |  | $1.5 |
Interest cost | 5.5 |  |  | 5.5 |  |  | 16.4 |  |  | 16.7 |  |  | 0.9 |  |  | 0.9 |  |  | 2.6 |  |  | 2.8 |
Expected return on plan assets | (7.2 | ) |  | (7.0 | ) |  | (21.4 | ) |  | (21.2 | ) |  | (0.2 | ) |  | (0.2 | ) |  | (0.6 | ) |  | (0.6 | )
Amortization of prior service cost (credit) | 0.1 |  |  | — |  |  | 0.1 |  |  | 0.1 |  |  | (0.1 | ) |  | (0.3 | ) |  | (0.2 | ) |  | (0.7 | )
Amortization of actuarial loss | 4.6 |  |  | 4.4 |  |  | 13.9 |  |  | 13.2 |  |  | 0.4 |  |  | 0.5 |  |  | 1.2 |  |  | 1.4 |
|  | $4.2 |  |  |  | $4.2 |  |  |  | $12.6 |  |  |  | $12.5 |  |  |  | $1.5 |  |  |  | $1.4 |  |  |  | $4.4 |  |  |  | $4.4 |
(a) | Settlement losses related to payments made to retired executives of Alliant Energy.
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
| 2017 |  | 2016 |  | 2017 |  | 2016 |  | 2017 |  | 2016 |  | 2017 |  | 2016 |  | 2017 |  | 2016 |  | 2017 |  | 2016
Compensation expense |  | $5.1 |  |  |  | $4.4 |  |  |  | $9.9 |  |  |  | $16.8 |  |  |  | $2.8 |  |  |  | $2.4 |  |  |  | $5.4 |  |  |  | $8.9 |  |  |  | $2.1 |  |  |  | $1.9 |  |  |  | $4.1 |  |  |  | $7.3 |
Income tax benefits | 2.1 |  |  | 1.7 |  |  | 4.0 |  |  | 6.8 |  |  | 1.1 |  |  | 1.0 |  |  | 2.2 |  |  | 3.7 |  |  | 0.9 |  |  | 0.7 |  |  | 1.7 |  |  | 2.9 |
Performance Shares and Performance Units [Member] |
Defined Benefit Plan Disclosure [Line Items] |
Schedule Of Equity-based Compensation Plans Activity | A summary of the performance shares and performance units activity for the nine months ended September 30, 2017, with amounts representing the target number of awards, was as follows:
| Performance Shares |  | Performance Units
Nonvested awards, January 1 | 257,599 |  |  | 93,320 |
Granted | 65,350 |  |  | 21,558 |
Vested | (99,438 | ) |  | (37,395 | )
Forfeited | — |  |  | (4,243 | )
Nonvested awards, September 30 | 223,511 |  |  | 73,240 |
Vested Awards - During the nine months ended September 30, 2017, certain performance shares and performance units that were granted in 2014 vested, resulting in payouts (a combination of cash and common stock for the performance shares and cash only for the performance units) as follows:
| Performance Shares |  | Performance Units
Performance awards vested | 99,438 |  |  | 37,395 |
Percentage of target number of performance awards | 147.5 | % |  | 147.5 | %
Aggregate payout value (in millions) |  | $5.6 |  |  |  | $1.5 |
Payout - cash (in millions) |  | $5.1 |  |  |  | $1.5 |
Payout - common stock shares issued | 5,185 |  |  | N/A |
Fair Value of Awards - Information related to fair values of nonvested performance shares and performance units at September 30, 2017, by year of grant, was as follows:
| Performance Shares |  | Performance Units
| 2017 Grant |  | 2016 Grant |  | 2015 Grant |  | 2017 Grant |  | 2016 Grant |  | 2015 Grant
Nonvested awards at target | 65,350 |  |  | 67,355 |  |  | 90,806 |  |  | 19,531 |  |  | 21,751 |  |  | 31,958 |
Alliant Energy common stock closing price on September 29, 2017 |  | $41.57 |  |  |  | $41.57 |  |  |  | $41.57 |  |  |  | $41.57 |  |  |  | $41.57 |  |  | N/A
Alliant Energy common stock closing price on grant date | N/A |  | N/A |  | N/A |  | N/A |  | N/A |  |  | $32.55 |
Estimated payout percentage based on performance criteria | 100 | % |  | 138 | % |  | 113 | % |  | 100 | % |  | 138 | % |  | 113 | %
Fair values of each nonvested award |  | $41.57 |  |  |  | $57.37 |  |  |  | $46.97 |  |  |  | $41.57 |  |  |  | $57.37 |  |  |  | $36.78 |
Performance Restricted Stock Unit [Member] |
Defined Benefit Plan Disclosure [Line Items] |
Schedule Of Equity-based Compensation Plans Activity | A summary of the performance restricted stock units activity for the nine months ended September 30, 2017, with amounts representing the target number of units, was as follows:
| Units |  | Weighted Average Grant Date Fair Value
Nonvested units, January 1 | 67,355 |  |  |  | $33.96 |
Granted | 65,350 |  |  | 39.12 |
Nonvested units, September 30 | 132,705 |  |  | 36.50 |
Restricted Stock Unit [Member] |
Defined Benefit Plan Disclosure [Line Items] |
Schedule Of Equity-based Compensation Plans Activity | A summary of the restricted stock units activity for the nine months ended September 30, 2017, was as follows:
Nonvested units, January 1 | 57,736 |
Granted | 56,013 |
Nonvested units, September 30 | 113,749 |
IPL [Member] |
Defined Benefit Plan Disclosure [Line Items] |
Defined Benefit Pension And Other Postretirement Benefits Plans | 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 in the tables below (in millions). In IPL’s and WPL’s tables below, the defined benefit pension plan amounts represent those respective amounts for their bargaining unit employees covered under the qualified plans that they sponsor, as well as amounts directly assigned to them related to their current and former non-bargaining employees who are participants in the Alliant Energy and Corporate Services sponsored qualified and non-qualified defined benefit pension plans. In IPL’s and WPL’s tables below, the OPEB plans amounts represent respective amounts for their employees, as well as amounts directly assigned to them related to their current and former non-bargaining employees who are participants in the Corporate Services sponsored OPEB plan.
| Defined Benefit Pension Plans |  | OPEB Plans
| Three Months |  | Nine Months |  | Three Months |  | Nine Months
Alliant Energy | 2017 |  | 2016 |  | 2017 |  | 2016 |  | 2017 |  | 2016 |  | 2017 |  | 2016
Service cost |  | $3.1 |  |  |  | $3.2 |  |  |  | $9.3 |  |  |  | $9.5 |  |  |  | $1.2 |  |  |  | $1.4 |  |  |  | $3.7 |  |  |  | $4.0 |
Interest cost | 12.7 |  |  | 13.2 |  |  | 38.3 |  |  | 39.7 |  |  | 2.2 |  |  | 2.3 |  |  | 6.5 |  |  | 7.0 |
Expected return on plan assets | (16.3 | ) |  | (16.3 | ) |  | (49.1 | ) |  | (49.1 | ) |  | (1.5 | ) |  | (1.6 | ) |  | (4.6 | ) |  | (4.6 | )
Amortization of prior service credit | (0.1 | ) |  | (0.1 | ) |  | (0.3 | ) |  | (0.2 | ) |  | (0.1 | ) |  | (1.0 | ) |  | (0.2 | ) |  | (3.1 | )
Amortization of actuarial loss | 9.4 |  |  | 9.3 |  |  | 28.2 |  |  | 28.0 |  |  | 1.0 |  |  | 1.2 |  |  | 2.9 |  |  | 3.6 |
Settlement losses (a) | 0.9 |  |  | — |  |  | 0.9 |  |  | — |  |  | — |  |  | — |  |  | — |  |  | — |
|  | $9.7 |  |  |  | $9.3 |  |  |  | $27.3 |  |  |  | $27.9 |  |  |  | $2.8 |  |  |  | $2.3 |  |  |  | $8.3 |  |  |  | $6.9 |
| Defined Benefit Pension Plans |  | OPEB Plans
| Three Months |  | Nine Months |  | Three Months |  | Nine Months
IPL | 2017 |  | 2016 |  | 2017 |  | 2016 |  | 2017 |  | 2016 |  | 2017 |  | 2016
Service cost |  | $1.8 |  |  |  | $1.8 |  |  |  | $5.5 |  |  |  | $5.6 |  |  |  | $0.5 |  |  |  | $0.5 |  |  |  | $1.6 |  |  |  | $1.7 |
Interest cost | 5.9 |  |  | 6.1 |  |  | 17.6 |  |  | 18.4 |  |  | 0.8 |  |  | 1.0 |  |  | 2.6 |  |  | 2.9 |
Expected return on plan assets | (7.7 | ) |  | (7.7 | ) |  | (23.1 | ) |  | (23.2 | ) |  | (1.0 | ) |  | (1.0 | ) |  | (3.2 | ) |  | (3.2 | )
Amortization of prior service credit | — |  |  | — |  |  | (0.1 | ) |  | (0.1 | ) |  | — |  |  | (0.7 | ) |  | — |  |  | (2.0 | )
Amortization of actuarial loss | 4.0 |  |  | 4.2 |  |  | 12.1 |  |  | 12.4 |  |  | 0.5 |  |  | 0.7 |  |  | 1.5 |  |  | 2.0 |
|  | $4.0 |  |  |  | $4.4 |  |  |  | $12.0 |  |  |  | $13.1 |  |  |  | $0.8 |  |  |  | $0.5 |  |  |  | $2.5 |  |  |  | $1.4 |
| Defined Benefit Pension Plans |  | OPEB Plans
| Three Months |  | Nine Months |  | Three Months |  | Nine Months
WPL | 2017 |  | 2016 |  | 2017 |  | 2016 |  | 2017 |  | 2016 |  | 2017 |  | 2016
Service cost |  | $1.2 |  |  |  | $1.3 |  |  |  | $3.6 |  |  |  | $3.7 |  |  |  | $0.5 |  |  |  | $0.5 |  |  |  | $1.4 |  |  |  | $1.5 |
Interest cost | 5.5 |  |  | 5.5 |  |  | 16.4 |  |  | 16.7 |  |  | 0.9 |  |  | 0.9 |  |  | 2.6 |  |  | 2.8 |
Expected return on plan assets | (7.2 | ) |  | (7.0 | ) |  | (21.4 | ) |  | (21.2 | ) |  | (0.2 | ) |  | (0.2 | ) |  | (0.6 | ) |  | (0.6 | )
Amortization of prior service cost (credit) | 0.1 |  |  | — |  |  | 0.1 |  |  | 0.1 |  |  | (0.1 | ) |  | (0.3 | ) |  | (0.2 | ) |  | (0.7 | )
Amortization of actuarial loss | 4.6 |  |  | 4.4 |  |  | 13.9 |  |  | 13.2 |  |  | 0.4 |  |  | 0.5 |  |  | 1.2 |  |  | 1.4 |
|  | $4.2 |  |  |  | $4.2 |  |  |  | $12.6 |  |  |  | $12.5 |  |  |  | $1.5 |  |  |  | $1.4 |  |  |  | $4.4 |  |  |  | $4.4 |
(a) | Settlement losses related to payments made to retired executives of Alliant Energy.
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
| 2017 |  | 2016 |  | 2017 |  | 2016 |  | 2017 |  | 2016 |  | 2017 |  | 2016 |  | 2017 |  | 2016 |  | 2017 |  | 2016
Compensation expense |  | $5.1 |  |  |  | $4.4 |  |  |  | $9.9 |  |  |  | $16.8 |  |  |  | $2.8 |  |  |  | $2.4 |  |  |  | $5.4 |  |  |  | $8.9 |  |  |  | $2.1 |  |  |  | $1.9 |  |  |  | $4.1 |  |  |  | $7.3 |
Income tax benefits | 2.1 |  |  | 1.7 |  |  | 4.0 |  |  | 6.8 |  |  | 1.1 |  |  | 1.0 |  |  | 2.2 |  |  | 3.7 |  |  | 0.9 |  |  | 0.7 |  |  | 1.7 |  |  | 2.9 |
WPL [Member] |
Defined Benefit Plan Disclosure [Line Items] |
Defined Benefit Pension And Other Postretirement Benefits Plans | 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 in the tables below (in millions). In IPL’s and WPL’s tables below, the defined benefit pension plan amounts represent those respective amounts for their bargaining unit employees covered under the qualified plans that they sponsor, as well as amounts directly assigned to them related to their current and former non-bargaining employees who are participants in the Alliant Energy and Corporate Services sponsored qualified and non-qualified defined benefit pension plans. In IPL’s and WPL’s tables below, the OPEB plans amounts represent respective amounts for their employees, as well as amounts directly assigned to them related to their current and former non-bargaining employees who are participants in the Corporate Services sponsored OPEB plan.
| Defined Benefit Pension Plans |  | OPEB Plans
| Three Months |  | Nine Months |  | Three Months |  | Nine Months
Alliant Energy | 2017 |  | 2016 |  | 2017 |  | 2016 |  | 2017 |  | 2016 |  | 2017 |  | 2016
Service cost |  | $3.1 |  |  |  | $3.2 |  |  |  | $9.3 |  |  |  | $9.5 |  |  |  | $1.2 |  |  |  | $1.4 |  |  |  | $3.7 |  |  |  | $4.0 |
Interest cost | 12.7 |  |  | 13.2 |  |  | 38.3 |  |  | 39.7 |  |  | 2.2 |  |  | 2.3 |  |  | 6.5 |  |  | 7.0 |
Expected return on plan assets | (16.3 | ) |  | (16.3 | ) |  | (49.1 | ) |  | (49.1 | ) |  | (1.5 | ) |  | (1.6 | ) |  | (4.6 | ) |  | (4.6 | )
Amortization of prior service credit | (0.1 | ) |  | (0.1 | ) |  | (0.3 | ) |  | (0.2 | ) |  | (0.1 | ) |  | (1.0 | ) |  | (0.2 | ) |  | (3.1 | )
Amortization of actuarial loss | 9.4 |  |  | 9.3 |  |  | 28.2 |  |  | 28.0 |  |  | 1.0 |  |  | 1.2 |  |  | 2.9 |  |  | 3.6 |
Settlement losses (a) | 0.9 |  |  | — |  |  | 0.9 |  |  | — |  |  | — |  |  | — |  |  | — |  |  | — |
|  | $9.7 |  |  |  | $9.3 |  |  |  | $27.3 |  |  |  | $27.9 |  |  |  | $2.8 |  |  |  | $2.3 |  |  |  | $8.3 |  |  |  | $6.9 |
| Defined Benefit Pension Plans |  | OPEB Plans
| Three Months |  | Nine Months |  | Three Months |  | Nine Months
IPL | 2017 |  | 2016 |  | 2017 |  | 2016 |  | 2017 |  | 2016 |  | 2017 |  | 2016
Service cost |  | $1.8 |  |  |  | $1.8 |  |  |  | $5.5 |  |  |  | $5.6 |  |  |  | $0.5 |  |  |  | $0.5 |  |  |  | $1.6 |  |  |  | $1.7 |
Interest cost | 5.9 |  |  | 6.1 |  |  | 17.6 |  |  | 18.4 |  |  | 0.8 |  |  | 1.0 |  |  | 2.6 |  |  | 2.9 |
Expected return on plan assets | (7.7 | ) |  | (7.7 | ) |  | (23.1 | ) |  | (23.2 | ) |  | (1.0 | ) |  | (1.0 | ) |  | (3.2 | ) |  | (3.2 | )
Amortization of prior service credit | — |  |  | — |  |  | (0.1 | ) |  | (0.1 | ) |  | — |  |  | (0.7 | ) |  | — |  |  | (2.0 | )
Amortization of actuarial loss | 4.0 |  |  | 4.2 |  |  | 12.1 |  |  | 12.4 |  |  | 0.5 |  |  | 0.7 |  |  | 1.5 |  |  | 2.0 |
|  | $4.0 |  |  |  | $4.4 |  |  |  | $12.0 |  |  |  | $13.1 |  |  |  | $0.8 |  |  |  | $0.5 |  |  |  | $2.5 |  |  |  | $1.4 |
| Defined Benefit Pension Plans |  | OPEB Plans
| Three Months |  | Nine Months |  | Three Months |  | Nine Months
WPL | 2017 |  | 2016 |  | 2017 |  | 2016 |  | 2017 |  | 2016 |  | 2017 |  | 2016
Service cost |  | $1.2 |  |  |  | $1.3 |  |  |  | $3.6 |  |  |  | $3.7 |  |  |  | $0.5 |  |  |  | $0.5 |  |  |  | $1.4 |  |  |  | $1.5 |
Interest cost | 5.5 |  |  | 5.5 |  |  | 16.4 |  |  | 16.7 |  |  | 0.9 |  |  | 0.9 |  |  | 2.6 |  |  | 2.8 |
Expected return on plan assets | (7.2 | ) |  | (7.0 | ) |  | (21.4 | ) |  | (21.2 | ) |  | (0.2 | ) |  | (0.2 | ) |  | (0.6 | ) |  | (0.6 | )
Amortization of prior service cost (credit) | 0.1 |  |  | — |  |  | 0.1 |  |  | 0.1 |  |  | (0.1 | ) |  | (0.3 | ) |  | (0.2 | ) |  | (0.7 | )
Amortization of actuarial loss | 4.6 |  |  | 4.4 |  |  | 13.9 |  |  | 13.2 |  |  | 0.4 |  |  | 0.5 |  |  | 1.2 |  |  | 1.4 |
|  | $4.2 |  |  |  | $4.2 |  |  |  | $12.6 |  |  |  | $12.5 |  |  |  | $1.5 |  |  |  | $1.4 |  |  |  | $4.4 |  |  |  | $4.4 |
(a) | Settlement losses related to payments made to retired executives of Alliant Energy.
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
| 2017 |  | 2016 |  | 2017 |  | 2016 |  | 2017 |  | 2016 |  | 2017 |  | 2016 |  | 2017 |  | 2016 |  | 2017 |  | 2016
Compensation expense |  | $5.1 |  |  |  | $4.4 |  |  |  | $9.9 |  |  |  | $16.8 |  |  |  | $2.8 |  |  |  | $2.4 |  |  |  | $5.4 |  |  |  | $8.9 |  |  |  | $2.1 |  |  |  | $1.9 |  |  |  | $4.1 |  |  |  | $7.3 |
Income tax benefits | 2.1 |  |  | 1.7 |  |  | 4.0 |  |  | 6.8 |  |  | 1.1 |  |  | 1.0 |  |  | 2.2 |  |  | 3.7 |  |  | 0.9 |  |  | 0.7 |  |  | 1.7 |  |  | 2.9 |

X
- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_DefinedBenefitPlanDisclosureLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Tabular disclosure of components of a stock option or other award plan under which equity-based compensation is awarded to employees, typically comprised of the amount of unearned compensation (deferred compensation cost), compensation expense, and changes in the quantity and fair value of the shares (or other type of equity) granted, exercised, forfeited, and issued and outstanding pertaining to that plan. Disclosure may also include nature and general terms of such arrangements that existed during the period and potential effects of those arrangements on shareholders, effect of compensation cost arising from equity-based payment arrangements on the income statement, method of estimating the fair value of the goods or services received, or the fair value of the equity instruments granted, during the period, cash flow effects resulting from equity-based payment arrangements and, for registrants that accelerate vesting of out of the money share options, reasons for the decision to accelerate. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5047-113901 + Details
Name: | us-gaap_DisclosureOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTextBlock
Namespace Prefix: | us-gaap_
Data Type: | nonnum:textBlockItemType
Balance Type: | na
Period Type: | duration

X
- Definition Tabular disclosure of the allocation of equity-based compensation costs to a given line item on the balance sheet and income statement for the period. This may include the reporting line for the costs and the amount capitalized and expensed. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5047-113901 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB Topic 14.F) -URI http://asc.fasb.org/extlink&oid=27013229&loc=d3e301413-122809 + Details
Name: | us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTextBlock
Namespace Prefix: | us-gaap_
Data Type: | nonnum:textBlockItemType
Balance Type: | na
Period Type: | duration

X
- Definition Tabular disclosure of the components of net benefit costs for pension plans and/or other employee benefit plans including service cost, interest cost, expected return on plan assets, gain (loss), prior service cost or credit, transition asset or obligation, and gain (loss) recognized due to settlements or curtailments. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h) -URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e1928-114920 + Details
Name: | us-gaap_ScheduleOfNetBenefitCostsTableTextBlock
Namespace Prefix: | us-gaap_
Data Type: | nonnum:textBlockItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_AwardTypeAxis=lnt_PerformanceSharesAndUnitsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_AwardTypeAxis=lnt_PerformanceRestrictedStockUnitMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_AwardTypeAxis=lnt_RestrictedStockUnitAndRestrictedUnitMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | dei_LegalEntityAxis=lnt_IplMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | dei_LegalEntityAxis=lnt_WplMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |