Fair Value of Financial Assets and Liabilities, Impact of Derivative Activity (Details) - USD ($)	3 Months Ended
Mar. 31, 2017	Mar. 31, 2016
Financial Impact of Qualifying Fair Value Hedges on Earnings [Abstract]
Derivative instruments designated as fair value hedges	$ 0	$ 0
Recognized gains (losses) from fair value hedges or related hedged transactions	0	0
Other Derivative Instruments \| Natural Gas Commodity
Derivative Instruments, Gain (Loss) [Line Items]
Pre-tax fair value gains (losses) recognized during the period in regulatory (assets) and liabilities	(100,000)	(100,000)
Pre-tax (gains) losses reclassified into income during the period from regulatory assets and (liabilities)	$ 200,000	$ 600,000

Derivative Instruments Gain Loss Reclassified To Regulatory Assets And Liabilities Net

No definition available.

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- Definition

-- None. No documentation exists for this element. --

+ References

No definition available.

+ Details

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- Definition

Pre-tax (gains) losses reclassified into income during the period from regulatory assets and (liabilities)

+ References

No definition available.

+ Details

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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

Net fair value of all derivative instruments designated as fair value hedging instruments.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958

+ Details

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- Definition

Total amount of gain (loss) derived from fair value hedges recognized in earnings in the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4C
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=64837297&loc=SL5624171-113959

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