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Summary of Accounting Policies Acquisitions Additional information (Details) | 12 Months Ended
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Dec. 31, 2017 USD ($)
Business Acquisition [Line Items] |
Number of customers acquired through acquisition | 800
ARM Energy [Member] |
Business Acquisition [Line Items] |
Business Combination, Liabilities Arising from Contingencies, Amount Recognized | $ 2,500,000.0
Central Gas [Member] |
Business Acquisition [Line Items] |
Number of customers acquired through acquisition | 325
Unregulated Energy [Member] | ARM [Member] |
Business Acquisition [Line Items] |
Goodwill, Acquired During Period | $ 6,800,000

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