Regulatory Assets and Liabilities (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Regulatory Assets [Line Items]
Current regulatory assets	$ 3,299us-gaap_RegulatoryAssetsCurrent	$ 1,300us-gaap_RegulatoryAssetsCurrent
Noncurrent regulatory assets	49,923us-gaap_RegulatoryAssetsNoncurrent	58,141us-gaap_RegulatoryAssetsNoncurrent
Total regulatory assets	53,222us-gaap_RegulatoryAssets	59,441us-gaap_RegulatoryAssets
Regulatory Liabilities [Line Items]
Noncurrent regulatory liabilities	23,375us-gaap_RegulatoryLiabilityNoncurrent	19,252us-gaap_RegulatoryLiabilityNoncurrent
Total regulatory liabilities	27,379us-gaap_RegulatoryLiabilities	19,890us-gaap_RegulatoryLiabilities
Fuel recovery [Member]
Regulatory Liabilities [Line Items]
Current regulatory liabilities	4,004	638
Postretirement benefits [Member]
Regulatory Liabilities [Line Items]
Noncurrent regulatory liabilities	23,375	19,047
Other [Member]
Regulatory Liabilities [Line Items]
Noncurrent regulatory liabilities	0	205
Environmental costs [Member]
Regulatory Assets [Line Items]
Current regulatory assets	1,300	1,300
Noncurrent regulatory assets	2,109	2,843
Levelized depreciation [Member]
Regulatory Assets [Line Items]
Current regulatory assets	1,999	0
Noncurrent regulatory assets	29,775	33,261
Grossed-up deferred taxes on equity funds used during construction [Member]
Regulatory Assets [Line Items]
Noncurrent regulatory assets	14,567	15,325
Asset retirement obligations, net [Member]
Regulatory Assets [Line Items]
Noncurrent regulatory assets	$ 3,472	$ 6,712

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The amount for the individual regulatory asset as itemized in a table of regulatory assets as of the end of the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 980

-SubTopic 340

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382

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- Definition

Carrying amount as of the balance sheet date of capitalized costs of regulated entities that are expected to be recovered through revenue sources within one year or the normal operating cycle, if longer. Such costs are capitalized if they meet both of the following criteria: a. It is probable that future revenue in an amount at least equal to the capitalized cost will result from inclusion of that cost in allowable costs for rate-making purposes. b. Based on available evidence, the future revenue will be provided to permit recovery of the previously incurred cost rather than to provide for expected levels of similar future costs. If the revenue will be provided through an automatic rate-adjustment clause, this criterion requires that the regulator's intent clearly be to permit recovery of the previously incurred cost.

+ References

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Carrying amount as of the balance sheet date of capitalized costs of regulated entities that are not expected to be recovered through revenue sources within one year or the normal operating cycle if longer.

+ References

+ Details

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The amount for the individual regulatory liability as itemized in a table of regulatory liabilities as of the end of the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 980

-SubTopic 405

-Section 25

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6500807&loc=d3e48068-110394

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The amount for the individual regulatory current liability as itemized in a table of regulatory current liabilities as of the end of the period.

+ References

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- Definition

The amount for the individual regulatory noncurrent liability as itemized in a table of regulatory noncurrent liabilities as of the end of the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.24)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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