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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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