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Regulatory Matters - Electric Rates (Details) - Retail Customers [Member] - USD ($) $ in Millions |  |  | 3 Months Ended |
--- | --- | --- | --- | ---
Dec. 20, 2018 | Jan. 18, 2018 | Mar. 31, 2019 | Mar. 31, 2018 | Dec. 31, 2018
PSCW [Member] | 2018 Wisconsin General Rate Case [Member] | SWL&P [Member] |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |
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] |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |
Revenue from Cost Recovery Riders |  |  | 7.4 | $ 24.1 |
Electric Rates [Member] | MPUC [Member] | 2016 Minnesota General Rate Case [Member] | Minnesota Power [Member] |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |
Annual Additional Revenue Generated from Requested Final Rate Increase |  | $ 13.0 |  |  |
Approved Return on Common Equity |  | 9.25% |  |  |
Approved Equity Ratio |  | 53.81% |  |  |
Reserve for Interim Rate Refund |  |  | $ 40.6 |  | $ 40.0

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