Regulatory Matters - Conservation Improvement Program (CIP) (Details) - CIP Consolidated Filing [Member] - MPUC [Member] - Minnesota Power [Member] - USD ($) $ in Millions		6 Months Ended
Jul. 19, 2019	Jun. 30, 2018
Regulatory Matters [Line Items]
CIP Financial Incentive		$ 3.0
Subsequent Event [Member]
Regulatory Matters [Line Items]
CIP Financial Incentive	$ 2.8

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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