MTPSC (Details 3) - MTPSC [Member] - USD ($)	Jan. 12, 2018	Sep. 25, 2017
Natural gas rate proceeding [Member]
Public Utilities, General Disclosures [Line Items]
Public utilities, requested rate increase (decrease), amount		$ 2,800,000
Public utilities, requested rate increase (decrease), percentage		4.10%
Public utilities, interim rate increase (decrease), amount		$ 1,600,000
Public utilities, interim rate increase (decrease), percentage		2.30%
Subsequent event [Member]
Public Utilities, General Disclosures [Line Items]
Public utilities interim rate increase (decrease), revised	$ 764,000

Public utilities interim rate increase (decrease), revised

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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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