Regulatory Matters - Conservation Improvement Program (CIP) (Details) - USD ($) $ in Millions	9 Months Ended
Sep. 30, 2019	Sep. 30, 2018
CIP Consolidated Filing [Member] \| MPUC [Member] \| Minnesota Power [Member]
Regulatory Matters [Line Items]
CIP Financial Incentive	$ 2.8	$ 3.0

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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