Goodwill (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2016	Dec. 31, 2015
Goodwill [Line Items]
Change in goodwill	$ 0
Goodwill	357,586	$ 357,586
Electric [Member]
Goodwill [Line Items]
Goodwill	243,558	243,558
Natural gas [Member]
Goodwill [Line Items]
Goodwill	$ 114,028	$ 114,028

Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6388280&loc=d3e13770-109266

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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- Definition

Amount of increase (decrease) of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

+ References

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- Details

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- Details

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