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Regulatory Matters - Electric Rates (Details) $ in Millions |  |  |  |  |  | 1 Months Ended | 3 Months Ended | 6 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
May 25, 2018 USD ($) | Apr. 02, 2018 USD ($) | Jan. 18, 2018 USD ($) | Aug. 09, 2017 | Apr. 13, 2017 USD ($) | Dec. 31, 2016 USD ($) | Nov. 30, 2016 USD ($) | Sep. 30, 2018 USD ($) Customers Years MW | Sep. 30, 2017 USD ($) | Jun. 30, 2018 USD ($) | Sep. 30, 2018 USD ($) Customers Years MW | Sep. 30, 2017 USD ($) | Dec. 31, 2017 USD ($)
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Pre-Tax Charge to Fuel, Purchase Power and Gas - Utility |  |  |  |  |  |  |  | $ 103.2 | $ 93.5 |  | $ 300.6 | $ 283.2 |
MPUC [Member] | 2016 Minnesota General Rate Case [Member] | Minnesota Power [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Pre-Tax Charge to Fuel, Purchase Power and Gas - Utility |  |  |  |  |  |  |  |  |  |  |  |  | $ 19.5
MPUC [Member] | 2016 Minnesota General Rate Case [Member] | Minnesota Power [Member] | Boswell Energy Center [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Expected Decrease in Annual Depreciation Expense |  |  |  |  |  |  |  | 25.0 |  |  | 25.0 |  |
PSCW [Member] | 2016 Wisconsin General Rate Case [Member] | SWL&P [Member] | Retail Customers [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Approved Return on Common Equity |  |  |  | 10.50% |  |  |  |  |  |  |  |  |
Approved Equity Ratio |  |  |  | 55.00% |  |  |  |  |  |  |  |  |
Approved Rate Increase Amount |  |  |  |  |  |  |  |  |  | $ 2.5 |  |  |
PSCW [Member] | 2012 Wisconsin General Rate Case [Member] | SWL&P [Member] | Retail Customers [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Approved Return on Common Equity |  |  |  |  |  |  |  |  |  | 10.90% |  |  |
PSCW [Member] | 2018 Wisconsin General Rate Case [Member] | SWL&P [Member] | Retail Customers [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Requested Average Rate Increase | 2.70% |  |  |  |  |  |  |  |  |  |  |  |
Requested Return on Equity | 10.50% |  |  |  |  |  |  |  |  |  |  |  |
Requested Equity Ratio | 55.41% |  |  |  |  |  |  |  |  |  |  |  |
Annual Additional Revenue Generated from Requested Final Rate Increase | $ 2.4 |  |  |  |  |  |  |  |  |  |  |  |
Electric Rates [Member] | MPUC [Member] | Minnesota Cost Recovery Riders [Member] | Minnesota Power [Member] | Retail Customers [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Revenue from Cost Recovery Riders |  |  |  |  |  |  |  | 29.1 | 23.1 |  | 81.2 | 71.7 |
Electric Rates [Member] | MPUC [Member] | 2016 Minnesota General Rate Case [Member] | Minnesota Power [Member] | Retail Customers [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Requested Average Rate Increase |  |  |  |  |  |  | 9.00% |  |  |  |  |  |
Requested Return on Equity |  |  |  |  |  |  | 10.25% |  |  |  |  |  |
Requested Equity Ratio |  |  |  |  |  |  | 53.81% |  |  |  |  |  |
Annual Additional Revenue Generated from Requested Final Rate Increase |  |  | $ 13.0 |  |  |  | $ 55.0 |  |  |  |  |  |
Annual Additional Revenue Generated from Requested Interim Rate Increase |  |  |  |  | $ 32.2 | $ 34.7 | 49.0 |  |  |  |  |  |
Annual Additional Revenue Generated from Requested Final Rate Increase, Amended |  |  |  |  |  |  | $ 49.0 |  |  |  |  |  |
Approved Return on Common Equity |  |  | 9.25% |  |  |  |  |  |  |  |  |  |
Approved Equity Ratio |  |  | 53.81% |  |  |  |  |  |  |  |  |  |
Reserve for Interim Rate Refund |  |  |  |  |  |  |  | 57.5 |  |  | 57.5 |  | 32.3
Electric Rates [Member] | MPUC [Member] | 2016 Minnesota General Rate Case [Member] | Minnesota Power [Member] | Retail Customers [Member] | Minimum [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Requested Reconsideration of Certain Decisions Representing Annualized Additional Revenue |  | $ 20.0 |  |  |  |  |  |  |  |  |  |  |
Electric Rates [Member] | MPUC [Member] | 2016 Minnesota General Rate Case [Member] | Minnesota Power [Member] | Retail Customers [Member] | Maximum [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Requested Reconsideration of Certain Decisions Representing Annualized Additional Revenue |  | $ 25.0 |  |  |  |  |  |  |  |  |  |  |
Electric Rates [Member] | MPUC [Member] | Energy-Intensive Trade-Exposed Customer Rates [Member] | Minnesota Power [Member] | Retail Customers [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Provided Discounts to EITE Customers |  |  |  |  |  |  |  | 4.1 | $ 2.7 |  | 12.2 | 8.6 | 8.6
Expected Discounts to EITE Customers Annually |  |  |  |  |  |  |  | $ 15.0 |  |  | $ 15.0 |  |
Electric Rates [Member] | MPUC [Member] | Renewable Cost Recovery Rider [Member] | Retail Customers [Member] | Corporate and Other [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
After-Tax Change for North Dakota Investment Tax Credits Reversed |  |  |  |  |  |  |  |  |  |  |  | 7.9 | 7.9
Electric Rates [Member] | MPUC [Member] | Renewable Cost Recovery Rider [Member] | Minnesota Power [Member] | Retail Customers [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Approved Rate Increase Amount |  |  |  |  |  |  |  |  |  |  |  | $ 14.3 | $ 14.3
Electric Rates [Member] | MPUC [Member] | Tax Cuts and Jobs Act [Member] | Minnesota Power [Member] | Retail Customers [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Amortization Periord of the Income Tax Benefits from the Remeasurement of Deferred Income Tax Assets and Liabilities Resulting from the TCJA Excluding the Related Property Plant and Equipment |  |  |  |  |  |  |  |  |  |  | 10 years |  |
Electric Rates [Member] | FERC [Member] | FERC-Approved Wholesale Rates [Member] | Minnesota Power [Member] | Municipal Customers [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Number of Customers | Customers |  |  |  |  |  |  |  | 16 |  |  | 16 |  |
Length of Notice Required to Terminate (Years) | Years |  |  |  |  |  |  |  | 3 |  |  | 3 |  |
Electric Rates [Member] | FERC [Member] | FERC-Approved Wholesale Rates [Member] | Minnesota Power [Member] | Municipal Customers [Member] | Wholesale Electric Contract (Termination Effective June 2019) [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Average Monthly Demand (MW) | MW |  |  |  |  |  |  |  | 29 |  |  | 29 |  |
Electric Rates [Member] | FERC [Member] | FERC-Approved Wholesale Rates [Member] | Minnesota Power [Member] | Municipal Customers [Member] | Wholesale Electric Contract (Cost-Based Formula Methodology for Entire Term) [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Number of Customers | Customers |  |  |  |  |  |  |  | 3 |  |  | 3 |  |
Electric Rates [Member] | FERC [Member] | FERC-Approved Wholesale Rates [Member] | Minnesota Power [Member] | Municipal Customers [Member] | Wholesale Electric Contracts (Expire from 2024 through 2029) [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Number of Customers | Customers |  |  |  |  |  |  |  | 14 |  |  | 14 |  |
Electric Rates [Member] | PSCW [Member] | 2018 Wisconsin General Rate Case [Member] | SWL&P [Member] | Retail Customers [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Requested Average Rate Increase | 2.00% |  |  |  |  |  |  |  |  |  |  |  |
Natural Gas [Member] | PSCW [Member] | 2018 Wisconsin General Rate Case [Member] | SWL&P [Member] | Retail Customers [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Requested Average Rate Increase | 2.30% |  |  |  |  |  |  |  |  |  |  |  |
Water [Member] | PSCW [Member] | 2018 Wisconsin General Rate Case [Member] | SWL&P [Member] | Retail Customers [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Requested Average Rate Increase | 8.30% |  |  |  |  |  |  |  |  |  |  |  |

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