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Regulatory Matters - Electric Rates (Details) $ in Millions			1 Months Ended		5 Months Ended	7 Months Ended	12 Months Ended
	Jan. 18, 2018 USD ($)	Apr. 13, 2017 USD ($)	Dec. 31, 2016 USD ($)	Nov. 30, 2016 USD ($)	Dec. 31, 2017 USD ($) Customers Years MW	Aug. 13, 2017	Dec. 31, 2017 USD ($) Customers Years MW	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Regulatory Matters [Line Items]
Charge to Fuel, Purchased Power and Gas - Utility							$ 396.9	$ 339.9	$ 336.0
Reduction in Operating Revenue due to Regulatory Outcome							1,063.8	1,000.7	991.2
Energy-Intensive Trade-Exposed Customer Rates [Member] | Minnesota Power [Member]
Regulatory Matters [Line Items]
Discount Provided to EITE Customers							8.6
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		25.0
PSCW [Member] | 2016 Wisconsin General Rate Case [Member] | SWL&P [Member] | Retail Customers [Member]
Regulatory Matters [Line Items]
Requested Return on Equity						10.90%
Annual Additional Revenue Generated from Requested Final Rate Increase							2.5
Approved Return on Common Equity					10.50%
Approved Equity Ratio					55.00%
Approved Rate Increase (Decrease), Percentage					2.90%
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							$ 96.9	97.1	$ 89.6
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							53.81%
Annual Additional Revenue Generated from Requested Final Rate Increase							$ 55.0
Annual Additional Revenue Generated from Requested Interim Rate Increase		$ 32.2	$ 34.7	$ 49.0			34.7
Requested Rate Increase, Amended, Amount				$ 49.0
Reserve for Interim Rate Refund					$ 32.0		32.0
Electric Rates [Member] | MPUC [Member] | 2016 Minnesota General Rate Case [Member] | Minnesota Power [Member] | Retail Customers [Member] | Subsequent Event [Member]
Regulatory Matters [Line Items]
Approved Return on Common Equity	9.25%
Approved Equity Ratio	53.81%
Approved Rate Increase - Amount	$ 13.0
Electric Rates [Member] | MPUC [Member] | Energy-Intensive Trade-Exposed Customer Rates [Member] | Minnesota Power [Member] | Retail Customers [Member]
Regulatory Matters [Line Items]
Discount Provided to EITE Customers							8.6
Estimated Discount to be Provided During the Next Twelve Months					$ 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	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
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		16
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 Contracts (expire December 2024) [Member]
Regulatory Matters [Line Items]
Number of Customers | Customers					14		14
Electric Rates [Member] | PSCW [Member] | 2016 Wisconsin General Rate Case [Member] | SWL&P [Member] | Retail Customers [Member]
Regulatory Matters [Line Items]
Approved Rate Increase (Decrease), Percentage					3.80%
Natural Gas Rates [Member] | PSCW [Member] | 2016 Wisconsin General Rate Case [Member] | SWL&P [Member] | Retail Customers [Member]
Regulatory Matters [Line Items]
Approved Rate Increase (Decrease), Percentage					4.80%
Water Rates [Member] | PSCW [Member] | 2016 Wisconsin General Rate Case [Member] | SWL&P [Member] | Retail Customers [Member]
Regulatory Matters [Line Items]
Approved Rate Increase (Decrease), Percentage					9.80%

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