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Summary of Significant Accounting Policies Nature of Operations (Details)	Dec. 31, 2017
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%

X

- Definition

The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=66011266&loc=d3e33918-111571

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- Definition

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+ References

No definition available.

+ Details

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X

- Definition

The number of customers served by the entity.

+ References

No definition available.

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