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Regulatory Matters (Narrative) (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Regulatory Matters [Line Items] |  |  |  |
Increase (decrease) in regulatory liabilities |  | $ 83.8 | $ 63.0 | $ 67.8
Depreciation and amortization |  | 461.8 | 411.6 | 401.3
Regulatory liabilities |  | 1,497.2 | 681.0 |
Regulatory assets |  | 1,666.7 | 1,915.1 |
Commodity cost recovery [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Regulatory liabilities |  | 21.0 | 30.8 |
IPL [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Regulatory assets not earning a return |  | 56.0 |  |
Increase (decrease) in regulatory liabilities |  | 71.2 | 67.3 | 75.5
Other operation and maintenance |  | 403.8 | 383.7 | 389.9
Depreciation and amortization |  | 245.0 | 210.8 | 207.2
Regulatory liabilities |  | 755.4 | 430.8 |
Regulatory assets |  | 1,231.6 | 1,458.8 |
IPL [Member] | Commodity cost recovery [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Regulatory liabilities |  | 14.6 | 17.8 |
WPL [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Regulatory assets not earning a return |  | 5.0 |  |
Other operation and maintenance |  | 249.0 | 219.8 | 235.4
Depreciation and amortization |  | 212.9 | 192.5 | $ 184.3
Regulatory liabilities |  | $ 741.8 | 250.2 |
Annual bandwidth for fuel-related costs |  | 2.00% |  |
Regulatory assets |  | $ 435.1 | 456.3 |
WPL [Member] | Commodity cost recovery [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Regulatory liabilities |  | 6.4 | 13.0 |
Alliant Energy and IPL [Member] | IPL's tax benefit riders [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Increase (decrease) in regulatory liabilities |  | $ 17.0 |  |
Scenario, Forecast [Member] | IPL [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Inflation-adjusted average of actual and estimated costs recovery period | 5 years |  |  |
Nelson Dewey Units 1 and 2 and Edgewater Unit 3 [Member] | WPL [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Expenditure collection period after new rate implementation |  | 10 years |  |
2016 Test Year Retail Electric [Member] | IPL [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Interim rate increase (decrease), amount |  | $ 102.0 |  |
Interim rate increase (decrease), percentage |  | 7.00% |  |
Write-down of regulatory assets |  | $ 9.0 |  |
2016 Test Year Retail Electric [Member] | Alliant Energy and IPL [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Increase in base rates |  | 77.0 |  |
2016 Test Year Retail Electric [Member] | Scenario, Forecast [Member] | IPL [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Settlement agreement rate increase, amount | $ 130.0 |  |  |
Settlement agreement rate increase, percentage | 9.00% |  |  |
2017/2018 Test Period Retail Electric [Member] | WPL [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Increase in base rates |  | 60.0 |  |
Authorized increase (decrease) in final rates, amount |  | 9.0 |  |
Reduction in fuel-related costs |  | 51.0 |  |
2017/2018 Test Period Retail Electric [Member] | Alliant Energy and WPL [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Increase in base rates |  | 63.0 |  |
2017/2018 Test Period Retail Gas [Member] | WPL [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Authorized increase (decrease) in final rates, amount |  | 9.0 |  |
2017/2018 Test Period Retail Gas [Member] | Alliant Energy and WPL [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Increase in base rates |  | 9.0 |  |
2015/2016 Test Period Retail Electric And Gas [Member] | WPL [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Authorized increase (decrease) in final rates, amount |  | (5.0) |  |
2015/2016 Test Period Retail Electric And Gas [Member] | Alliant Energy and WPL [Member] | Deferral [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Regulatory liabilities |  | 5.0 |  |
2016 Test Year Retail Electric Fuel Related [Member] | WPL [Member] | Commodity cost recovery [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Regulatory liabilities |  | 3.0 |  |
Regulatory assets |  | $ 6.0 |  |
2018 Test Year Retail Electric Fuel Related [Member] | Scenario, Forecast [Member] | WPL [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Authorized increase (decrease) in final rates, amount | $ 6.0 |  |  |
Autorized increase in final rates, percentage | 1.00% |  |  |
FERC [Member] | MISO Transmission Owner Complaints [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Base return on equity for refund period from November 12, 2013 through February 11, 2015, percent |  | 10.32% |  |
Refund received, after final true-up |  | $ 50.0 |  |
FERC [Member] | MISO Transmission Owner Complaints [Member] | IPL [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Refund received, after final true-up |  | 39.0 |  |
FERC [Member] | MISO Transmission Owner Complaints [Member] | WPL [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Refund received, after final true-up |  | 11.0 |  |
Forward Contracts [Member] | IUB [Member] | Scenario, Forecast [Member] | IPL [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Recovery period, authorized | 10 years |  |  |
Sutherland Units One And Three [Member] | 2016 Test Year Retail Electric [Member] | IPL [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Other operation and maintenance |  | $ 4.0 |  |
Sutherland Units One And Three [Member] | IUB [Member] | Scenario, Forecast [Member] | IPL [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Recovery period, authorized | 10 years |  |  |
Sutherland Units One And Three [Member] | FERC [Member] | IPL [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Recovery period, authorized |  | 10 years |  |
AROs [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Regulatory assets |  | $ 109.3 | 105.9 |
AROs [Member] | IPL [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Regulatory assets |  | 72.5 | 64.3 |
AROs [Member] | WPL [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Regulatory assets |  | 36.8 | 41.6 |
AROs [Member] | 2016 Test Year Retail Electric [Member] | IPL [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Depreciation and amortization |  | 5.0 |  |
AROs [Member] | IUB [Member] | Scenario, Forecast [Member] | IPL [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Recovery period, authorized | 4 years |  |  |
Other [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Regulatory assets |  | 96.6 | 76.6 |
Other [Member] | IPL [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Regulatory assets |  | 55.3 | 41.9 |
Other [Member] | WPL [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Regulatory assets |  | 41.3 | $ 34.7 |
Other [Member] | 2017 Test Year Retail Electric Fuel Related [Member] | Alliant Energy and WPL [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Regulatory assets |  | $ 5.0 |  |

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