Regulatory Matters - Conservation Improvement Program (CIP) (Details) - MPUC [Member] - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
CIP Triennial Filing [Member] \| Minnesota Power [Member]
Regulatory Matters [Line Items]
CIP Investment Goal	$ 10.3	$ 10.3	$ 7.3
CIP Actual Spending	9.0	8.1	7.4
CIP Investment Goal in 2019	10.5
CIP Consolidated Filing [Member] \| Minnesota Power [Member]
Regulatory Matters [Line Items]
CIP Financial Incentive	$ 3.0	$ 5.5	$ 7.5
Minimum [Member]
Regulatory Matters [Line Items]
CIP Spending Minimum - Percentage	1.50%

Conservation improvement program actual annual spend.

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Conservation improvement program Investment Goal approved by MPUC.

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Conservation improvement program Investment Goal for year two approved by the MPUC.

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- Definition

Minnesota Next Generation Act of 2007 introduced a spend goal of 1.5% of net gross operating revenues from service provided in the state on energy CIPs each year.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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The amount of regulated other operating revenues recognized during the period.

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