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MTPSC (Details 3) - MTPSC [Member] - USD ($) | Jan. 12, 2018 | Sep. 25, 2017
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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 |

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