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Summary of Significant Accounting Policies General Information (Details) | Dec. 31, 2016 | Jun. 29, 2015
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Electric |  |
Product Information |  |
Number of customers | 1,600,000 |
Natural gas |  |
Product Information |  |
Number of customers | 2,800,000 |
ATC |  |
Product Information |  |
Equity method investment, ownership interest (as a percent) | 60.00% | 26.20%

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