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Regulatory Matters (Narrative) (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Regulatory Matters [Line Items] |  |  |
Regulatory liabilities | $ 1,493.2 | $ 1,497.2 |
Refunded Federal Tax Reform benefits | 66.0 |  |
Depreciation and amortization | 506.9 | 461.8 | $ 411.6
Revenue Subject to Refund [Member] |  |  |
Regulatory Matters [Line Items] |  |  |
Regulatory liabilities | 20.0 |  |
IPL [Member] |  |  |
Regulatory Matters [Line Items] |  |  |
Regulatory assets not earning a return | 77.0 |  |
Regulatory liabilities | 754.9 | 755.4 |
Refunded Federal Tax Reform benefits | 25.0 |  |
Other operation and maintenance | 402.6 | 396.6 | 376.9
Depreciation and amortization | 283.5 | 245.0 | 210.8
IPL [Member] | Revenue Subject to Refund [Member] |  |  |
Regulatory Matters [Line Items] |  |  |
Regulatory liabilities | 15.0 |  |
WPL [Member] |  |  |
Regulatory Matters [Line Items] |  |  |
Regulatory assets not earning a return | 6.0 |  |
Regulatory liabilities | 738.3 | 741.8 |
Refunded Federal Tax Reform benefits | 41.0 |  |
Other operation and maintenance | 241.6 | 238.5 | 209.6
Depreciation and amortization | 219.4 | 212.9 | $ 192.5
WPL [Member] | Revenue Subject to Refund [Member] |  |  |
Regulatory Matters [Line Items] |  |  |
Regulatory liabilities | 5.0 |  |
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% |
Authorized increase (decrease) in final rates, amount | $ 130.0 |  |
Authorized increase (decrease) in final rates, percentage | 9.00% |  |
Write-down of regulatory assets |  | $ 9.0 |
2017/2018 Test Period Retail Electric [Member] | WPL [Member] |  |  |
Regulatory Matters [Line Items] |  |  |
Authorized increase (decrease) in final rates, amount |  | 9.0 |
Increase in base rates |  | 60.0 |
Reduction in fuel-related costs |  | 51.0 |
2017/2018 Test Period Retail Electric [Member] | WPL [Member] | Revenue Subject to Refund [Member] |  |  |
Regulatory Matters [Line Items] |  |  |
Regulatory liabilities | $ 20.0 |  |
2017/2018 Test Period Retail Gas [Member] | WPL [Member] |  |  |
Regulatory Matters [Line Items] |  |  |
Authorized increase (decrease) in final rates, amount |  | 9.0 |
Forward Contracts [Member] | IUB [Member] | IPL [Member] |  |  |
Regulatory Matters [Line Items] |  |  |
Recovery period, authorized | 10 years |  |
Sutherland Units 1 And 3 [Member] | 2016 Test Year Retail Electric [Member] | IPL [Member] |  |  |
Regulatory Matters [Line Items] |  |  |
Other operation and maintenance |  | 4.0 |
Sutherland Units 1 And 3 [Member] | IUB [Member] | IPL [Member] |  |  |
Regulatory Matters [Line Items] |  |  |
Recovery period, authorized | 10 years |  |
AROs [Member] | 2016 Test Year Retail Electric [Member] | IPL [Member] |  |  |
Regulatory Matters [Line Items] |  |  |
Depreciation and amortization |  | $ 5.0 |
AROs [Member] | IUB [Member] | IPL [Member] |  |  |
Regulatory Matters [Line Items] |  |  |
Recovery period, authorized | 4 years |  |

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