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Regulatory Matters - Electric Rates (Details) $ in Millions |  |  |  |  | 3 Months Ended | 8 Months Ended | 12 Months Ended
--- | --- | --- | --- | --- | --- | --- | ---
Feb. 28, 2017 USD ($) | Dec. 12, 2016 USD ($) | Nov. 02, 2016 USD ($) | Jun. 28, 2016 USD ($) | Mar. 31, 2017 USD ($) Customers Years MW | Mar. 31, 2016 USD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($)
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] | 2016 Wisconsin General Rate Case [Member] | SWL&P [Member] | Retail Customers [Member] |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |
Requested Rate Increase (Decrease) |  |  |  | 3.10% |  |  |  |
Requested Return on Equity |  |  |  | 10.90% |  |  |  |
Requested Equity Ratio |  |  |  | 55.00% |  |  |  |
Annual Additional Revenue Generated from Requested Final Rate Increase |  |  |  | $ 2.7 |  |  |  |
Electric Rates [Member] | MPUC [Member] | 2010 Minnesota Rate Case [Member] | Minnesota Power [Member] | Retail Customers [Member] |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |
Approved Return on Common Equity |  |  |  |  | 10.38% |  |  |
Approved Equity Ratio |  |  |  |  | 54.29% |  |  |
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 |  |  |  |  | $ 24.2 | $ 25.4 |  |
Electric Rates [Member] | MPUC [Member] | 2016 Minnesota General Rate Case [Member] | Minnesota Power [Member] | Retail Customers [Member] |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |
Requested Rate Increase (Decrease) |  |  | 9.00% |  |  |  |  |
Requested Return on Equity |  |  | 10.25% |  |  |  |  |
Requested Equity Ratio |  |  | 53.80% |  |  |  |  |
Annual Additional Revenue Generated from Requested Final Rate Increase | $ 39.0 |  | $ 55.0 |  |  |  |  |
Annual Additional Revenue Generated from Requested Interim Rate Increase |  | $ 34.7 | 49.0 |  | $ 34.7 |  |  |
Electric Rates [Member] | MPUC [Member] | 2016 Minnesota General Rate Case [Member] | Minnesota Power [Member] | Retail Customers [Member] | Boswell Energy Center [Member] |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |
Expected Decrease in Annual Depreciation Expense |  |  | $ 25.0 |  |  |  |  |
Electric Rates [Member] | MPUC [Member] | 2016 Minnesota General Rate Case [Member] | Minnesota Power [Member] | Retail Customers [Member] | Subsequent Event [Member] |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |
Annual Additional Revenue Generated from Requested Interim Rate Increase |  |  |  |  |  |  | $ 32.2 |
Electric Rates [Member] | MPUC [Member] | Renewable Cost Recovery Rider [Member] | Retail Customers [Member] | Corporate and Other [Member] |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |
After-Tax Charge for North Dakota Investment Tax Credits Reversed |  |  |  |  |  |  |  | $ 8.8
Electric Rates [Member] | MPUC [Member] | Renewable Cost Recovery Rider [Member] | Minnesota Power [Member] | Retail Customers [Member] |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |
Reduction in Operating Revenue due to Regulatory Outcome |  |  |  |  |  |  |  | $ 15.0
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 Contracts (Expire December 2024) [Member] |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |
Number of Customers | Customers |  |  |  |  | 14 |  |  |
Electric Rates [Member] | FERC [Member] | FERC-Approved Wholesale Rates [Member] | Minnesota Power [Member] | Municipal Customers [Member] | Wholesale Electric Contracts (Expire December 2024) [Member] | Maximum [Member] |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |
Change in Annual Capacity Charge Per Contract, Percentage |  |  |  |  | 2.00% |  |  |
Electric Rates [Member] | FERC [Member] | FERC-Approved Wholesale Rates [Member] | Minnesota Power [Member] | Municipal Customers [Member] | Wholesale Electric Contracts (Expire December 2024) [Member] | Minimum [Member] |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |
Change in Annual Capacity Charge Per Contract, Percentage |  |  |  |  | (1.00%) |  |  |
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] | PSCW [Member] | 2016 Wisconsin General Rate Case [Member] | SWL&P [Member] | Retail Customers [Member] |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |
Requested Rate Increase (Decrease) |  |  |  | 3.50% |  |  |  |
Natural Gas Rates [Member] | PSCW [Member] | 2016 Wisconsin General Rate Case [Member] | SWL&P [Member] | Retail Customers [Member] |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |
Requested Rate Increase (Decrease) |  |  |  | (1.30%) |  |  |  |
Water Rates [Member] | PSCW [Member] | 2016 Wisconsin General Rate Case [Member] | SWL&P [Member] | Retail Customers [Member] |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |
Requested Rate Increase (Decrease) |  |  |  | 7.80% |  |  |  |

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