Regulatory Matters - Electric Rates (Details) $ in Millions						1 Months Ended		3 Months Ended		6 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 ($)	Nov. 30, 2016 USD ($)	Jun. 30, 2018 USD ($) Customers Years MW	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) Customers Years MW	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Regulatory Matters [Line Items]
Pre-Tax Charge to Fuel, Purchase Power and Gas - Utility								$ 96.5	$ 93.1	$ 197.4	$ 189.7
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%
Expected Annualized Collection of Additional Revenue Under Retail Rate Order										$ 2.5
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%
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								28.0	24.4	$ 52.1	48.6
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								49.2		49.2		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								3.8	$ 3.6	8.1	$ 5.9	$ 8.6
Expected Discounts to EITE Customers Annually								$ 15.0		$ 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
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%