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Commitments And Contingencies (Other Purchase Obligations) (Details) - USD ($)
$ in Millions
 1 Months Ended 12 Months Ended
Sep. 30, 2020
Jul. 31, 2018
Dec. 31, 2018
Long-term Purchase Commitment [Line Items]      
Minimum future commitments     $ 2,192
Individual commitments incurred     1
Purchased Power [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments [1]     1,045
Natural Gas [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments     881
Coal [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments [2]     195
Other [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments [3]     71
2019 [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments     575
2019 [Member] | Purchased Power [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments [1]     159
2019 [Member] | Natural Gas [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments     254
2019 [Member] | Coal [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments [2]     102
2019 [Member] | Other [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments [3]     60
2020 [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments     339
2020 [Member] | Purchased Power [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments [1]     135
2020 [Member] | Natural Gas [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments     151
2020 [Member] | Coal [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments [2]     49
2020 [Member] | Other [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments [3]     4
2021 [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments     297
2021 [Member] | Purchased Power [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments [1]     149
2021 [Member] | Natural Gas [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments     119
2021 [Member] | Coal [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments [2]     26
2021 [Member] | Other [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments [3]     3
2022 [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments     245
2022 [Member] | Purchased Power [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments [1]     140
2022 [Member] | Natural Gas [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments     94
2022 [Member] | Coal [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments [2]     9
2022 [Member] | Other [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments [3]     2
2023 [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments     233
2023 [Member] | Purchased Power [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments [1]     155
2023 [Member] | Natural Gas [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments     67
2023 [Member] | Coal [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments [2]     9
2023 [Member] | Other [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments [3]     2
Thereafter [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments     503
Thereafter [Member] | Purchased Power [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments [1]     307
Thereafter [Member] | Natural Gas [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments     196
Thereafter [Member] | Coal [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments [2]     0
Thereafter [Member] | Other [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments [3]     0
IPL [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments     1,546
IPL [Member] | Purchased Power [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments [1]     1,030
IPL [Member] | Natural Gas [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments     363
IPL [Member] | Coal [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments [2]     119
IPL [Member] | Other [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments [3]     34
IPL [Member] | 2019 [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments     344
IPL [Member] | 2019 [Member] | Purchased Power [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments [1]     144
IPL [Member] | 2019 [Member] | Natural Gas [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments     124
IPL [Member] | 2019 [Member] | Coal [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments [2]     52
IPL [Member] | 2019 [Member] | Other [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments [3]     24
IPL [Member] | 2020 [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments     222
IPL [Member] | 2020 [Member] | Purchased Power [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments [1]     135
IPL [Member] | 2020 [Member] | Natural Gas [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments     55
IPL [Member] | 2020 [Member] | Coal [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments [2]     29
IPL [Member] | 2020 [Member] | Other [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments [3]     3
IPL [Member] | 2021 [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments     214
IPL [Member] | 2021 [Member] | Purchased Power [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments [1]     149
IPL [Member] | 2021 [Member] | Natural Gas [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments     42
IPL [Member] | 2021 [Member] | Coal [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments [2]     20
IPL [Member] | 2021 [Member] | Other [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments [3]     3
IPL [Member] | 2022 [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments     184
IPL [Member] | 2022 [Member] | Purchased Power [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments [1]     140
IPL [Member] | 2022 [Member] | Natural Gas [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments     33
IPL [Member] | 2022 [Member] | Coal [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments [2]     9
IPL [Member] | 2022 [Member] | Other [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments [3]     2
IPL [Member] | 2023 [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments     192
IPL [Member] | 2023 [Member] | Purchased Power [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments [1]     155
IPL [Member] | 2023 [Member] | Natural Gas [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments     26
IPL [Member] | 2023 [Member] | Coal [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments [2]     9
IPL [Member] | 2023 [Member] | Other [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments [3]     2
IPL [Member] | Thereafter [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments     390
IPL [Member] | Thereafter [Member] | Purchased Power [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments [1]     307
IPL [Member] | Thereafter [Member] | Natural Gas [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments     83
IPL [Member] | Thereafter [Member] | Coal [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments [2]     0
IPL [Member] | Thereafter [Member] | Other [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments [3]     0
WPL [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments     632
WPL [Member] | Purchased Power [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments     15
WPL [Member] | Natural Gas [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments     518
WPL [Member] | Coal [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments [2]     76
WPL [Member] | Other [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments [3]     23
WPL [Member] | 2019 [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments     218
WPL [Member] | 2019 [Member] | Purchased Power [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments     15
WPL [Member] | 2019 [Member] | Natural Gas [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments     130
WPL [Member] | 2019 [Member] | Coal [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments [2]     50
WPL [Member] | 2019 [Member] | Other [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments [3]     23
WPL [Member] | 2020 [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments     116
WPL [Member] | 2020 [Member] | Purchased Power [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments     0
WPL [Member] | 2020 [Member] | Natural Gas [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments     96
WPL [Member] | 2020 [Member] | Coal [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments [2]     20
WPL [Member] | 2020 [Member] | Other [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments [3]     0
WPL [Member] | 2021 [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments     83
WPL [Member] | 2021 [Member] | Purchased Power [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments     0
WPL [Member] | 2021 [Member] | Natural Gas [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments     77
WPL [Member] | 2021 [Member] | Coal [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments [2]     6
WPL [Member] | 2021 [Member] | Other [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments [3]     0
WPL [Member] | 2022 [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments     61
WPL [Member] | 2022 [Member] | Purchased Power [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments     0
WPL [Member] | 2022 [Member] | Natural Gas [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments     61
WPL [Member] | 2022 [Member] | Coal [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments [2]     0
WPL [Member] | 2022 [Member] | Other [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments [3]     0
WPL [Member] | 2023 [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments     41
WPL [Member] | 2023 [Member] | Purchased Power [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments     0
WPL [Member] | 2023 [Member] | Natural Gas [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments     41
WPL [Member] | 2023 [Member] | Coal [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments [2]     0
WPL [Member] | 2023 [Member] | Other [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments [3]     0
WPL [Member] | Thereafter [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments     113
WPL [Member] | Thereafter [Member] | Purchased Power [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments     0
WPL [Member] | Thereafter [Member] | Natural Gas [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments     113
WPL [Member] | Thereafter [Member] | Coal [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments [2]     0
WPL [Member] | Thereafter [Member] | Other [Member]      
Long-term Purchase Commitment [Line Items]      
Minimum future commitments [3]     $ 0
DAEC PPA [Member]      
Long-term Purchase Commitment [Line Items]      
Long-term Contract for Purchase of Electric Power, Contract Term Amendment   5 years  
Scenario, Forecast [Member] | DAEC PPA [Member]      
Long-term Purchase Commitment [Line Items]      
Long-term Contract for Purchase of Electric Power, Buyout Payment $ 110    
[1] Includes payments required by PPAs for capacity rights and minimum quantities of MWhs required to be purchased. In July 2018, IPL entered into an amendment to shorten the term of the DAEC PPA by five years in exchange for a $110 million buyout payment by IPL in September 2020, subject to IUB approval. In December 2018, IPL received an order from the IUB, which was effective in January 2019, approving recovery of the buyout payment. As a result, Alliant Energy’s and IPL’s amounts in the above table do not include the September 2020 buyout payment, and the minimum future commitments reflect IPL’s purchase of capacity and the resulting energy from DAEC through December 2025, the original term of the PPA prior to the amendment. Amounts in the above table for 2021 and beyond relate to the DAEC PPA.
[2] Corporate Services entered into system-wide coal contracts on behalf of IPL and WPL that include minimum future commitments. These commitments were assigned to IPL and WPL based on information available as of December 31, 2018 regarding expected future usage, which is subject to change.
[3] Includes individual commitments incurred during the normal course of business that exceeded $1 million at December 31, 2018.