Montana Public Service Commission (MTPSC) (Details) - MTPSC - Electric rate proceeding $ in Millions	Sep. 28, 2018 USD ($)
Public Utilities, General Disclosures [Line Items]
Public utilities, requested rate increase (decrease), amount	$ 11.9
Public utilities, requested rate increase (decrease), percentage	18.90%
Public utilities, interim rate increase (decrease), amount	$ 4.6
Public utilities, interim rate increase (decrease), percentage	7.30%

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Amount of public utility's interim rate increase (decrease) allowed while waiting final decision of regulatory agency.

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Percentage of public utility's interim rate increase (decrease) allowed while waiting final decision of regulatory agency.

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Amount of public utility's requested rate increase (decrease) with regulatory agency.

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Percentage of public utility's requested rate increase (decrease) with regulatory agency.

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