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Regulatory Matters - Electric Rates (Details) $ in Millions |  |  |  |  | 1 Months Ended | 12 Months Ended
--- | --- | --- | --- | --- | --- | ---
Dec. 20, 2018 USD ($) | May 25, 2018 | Jan. 18, 2018 USD ($) | Aug. 09, 2017 | Nov. 30, 2016 | Dec. 31, 2018 USD ($) Customers Years MW | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($)
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |
Charge to Fuel, Purchased Power and Gas - Utility |  |  |  |  |  | $ 407.5 | $ 396.9 | $ 339.9
MPUC [Member] | 2016 Minnesota General Rate Case [Member] | Minnesota Power [Member] |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |
Charge to Fuel, Purchased 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] |  |  |  |  |  |  |  |
Decrease in Annual Depreciation Expense Based on Regulatory Order |  |  |  |  |  | $ 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% |  |  |  |
PSCW [Member] | 2012 Wisconsin 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% |  |  |  |  |  |
Approved Return on Common Equity | 10.40% |  |  |  |  |  |  |
Approved Equity Ratio | 55.00% |  |  |  |  |  |  |
Approved Rate Increase - Amount | $ 1.3 |  |  |  |  |  |  |
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 |  |  |  |  |  | $ 103.8 | 96.9 | 97.1
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% |  |  |
Approved Return on Common Equity |  |  | 9.25% |  |  |  |  |
Approved Equity Ratio |  |  | 53.81% |  |  |  |  |
Annual Additional Revenue Generated from Requested Final Rate Increase |  |  | $ 13.0 |  |  |  |  |
Reserve for Interim Rate Refund |  |  |  |  |  | 40.0 | 23.7 |
Electric Rates [Member] | MPUC [Member] | Energy-Intensive Trade-Exposed Customer Rates [Member] | Minnesota Power [Member] | Retail Customers [Member] |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |
Estimated Discount to be Provided During the Next Twelve Months |  |  |  |  |  | 16.0 |  |
Discount Provided to EITE Customers |  |  |  |  |  | $ 16.7 | 8.6 | 0.0
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 | 8.8
Electric Rates [Member] | MPUC [Member] | Renewable Cost Recovery Rider [Member] | Minnesota Power [Member] | Retail Customers [Member] |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |
Approved Rate Increase - Amount |  |  |  |  |  |  | $ 14.0 | $ 15.0
Electric Rates [Member] | MPUC [Member] | Tax Cuts and Jobs Act [Member] | Minnesota Power [Member] | Retail Customers [Member] |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |
Amortization Period 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 |  |
Notice Required to Terminate (Years) | Years |  |  |  |  |  | 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 |  |
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 |  |
Electric Rates [Member] | FERC [Member] | FERC-Approved Wholesale Rates [Member] | Minnesota Power [Member] | Municipal Customers [Member] | Wholesale Electric Contracts (expire December 2024) [Member] |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |
Number of Customers | Customers |  |  |  |  |  | 14 |  |
Electric Rates [Member] | PSCW [Member] | Tax Cuts and Jobs Act [Member] | SWL&P [Member] | Retail Customers [Member] |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |
Amortization Period 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 | 4 years |  |  |  |  |  |  |
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 Rates [Member] | PSCW [Member] | 2018 Wisconsin General Rate Case [Member] | SWL&P [Member] | Retail Customers [Member] |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |
Requested Average Rate Increase |  | 2.30% |  |  |  |  |  |
Water Rates [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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