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Regulatory Matters - Utility Rates (Details) $ in Millions				9 Months Ended
	Nov. 02, 2016 USD ($)	Jun. 28, 2016 USD ($)	Jun. 01, 2011	Sep. 30, 2016 USD ($) Customers Years MW	Sep. 30, 2015 USD ($)
Minnesota Power [Member] | Bison Wind Energy Center [Member]
Regulatory Matters [Line Items]
Generating Capacity (MW) | MW				497
PSCW [Member] | 2012 Wisconsin Rate Case [Member] | SWL&P [Member] | Retail [Member]
Regulatory Matters [Line Items]
Approved Return on Common Equity				10.90%
PSCW [Member] | 2016 Wisconsin Rate Case [Member] | SWL&P [Member] | Retail [Member]
Regulatory Matters [Line Items]
Requested Rate Increase (Decrease)		3.10%
Requested Return on Equity		10.90%
Requested Equity Capital Structure		55.00%
Requested Debt Capital Structure		45.00%
Annual Additional Revenue Generated from Overall Requested Rate Increase		$ 2.7
Electric Rates [Member] | MPUC [Member] | 2010 Minnesota Rate Case [Member] | Minnesota Power [Member] | Retail [Member]
Regulatory Matters [Line Items]
Approved Return on Common Equity			10.38%	10.38%
Approved Equity Ratio				54.29%
Electric Rates [Member] | MPUC [Member] | Minnesota Cost Recovery Riders [Member] | Minnesota Power [Member] | Retail [Member]
Regulatory Matters [Line Items]
Revenue from Cost Recovery Riders				$ 73.9	$ 67.8
Electric Rates [Member] | MPUC [Member] | 2016 Minnesota Rate Case [Member] | Minnesota Power [Member] | Retail [Member] | Subsequent Event [Member]
Regulatory Matters [Line Items]
Requested Rate Increase (Decrease)	9.00%
Requested Return on Equity	10.25%
Requested Equity Capital Structure	53.80%
Requested Debt Capital Structure	46.20%
Annual Additional Revenue Generated from Overall Requested Rate Increase	$ 55.0
Expected Interim Rates	$ 49.0
Electric Rates [Member] | MPUC [Member] | Renewable Cost Recovery Rider [Member] | Retail [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 [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
Length of Notice Required to Terminate Contract (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]
Number of Customers | Customers				1
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 Rate Case [Member] | SWL&P [Member] | Retail [Member]
Regulatory Matters [Line Items]
Requested Rate Increase (Decrease)		3.50%
Natural Gas Rates [Member] | PSCW [Member] | 2016 Wisconsin Rate Case [Member] | SWL&P [Member] | Retail [Member]
Regulatory Matters [Line Items]
Requested Rate Increase (Decrease)		(1.30%)
Water Rates [Member] | PSCW [Member] | 2016 Wisconsin Rate Case [Member] | SWL&P [Member] | Retail [Member]
Regulatory Matters [Line Items]
Requested Rate Increase (Decrease)		7.80%

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 25
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=68052875&loc=d3e28446-109314

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
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-Section 25
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=68052875&loc=d3e28511-109314

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