SELECTED QUARTERLY INFORMATION (UNAUDITED) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended	12 Months Ended
Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Operating revenues	$ 151,363us-gaap_RegulatedOperatingRevenue	$ 171,512us-gaap_RegulatedOperatingRevenue	$ 149,782us-gaap_RegulatedOperatingRevenue	$ 179,673us-gaap_RegulatedOperatingRevenue	$ 149,058us-gaap_RegulatedOperatingRevenue	$ 157,486us-gaap_RegulatedOperatingRevenue	$ 136,646us-gaap_RegulatedOperatingRevenue	$ 151,140us-gaap_RegulatedOperatingRevenue	$ 652,330us-gaap_RegulatedOperatingRevenue	$ 594,330us-gaap_RegulatedOperatingRevenue	$ 557,097us-gaap_RegulatedOperatingRevenue
Operating income	19,300us-gaap_OperatingIncomeLoss	31,709us-gaap_OperatingIncomeLoss	19,502us-gaap_OperatingIncomeLoss	29,488us-gaap_OperatingIncomeLoss	23,799us-gaap_OperatingIncomeLoss	32,896us-gaap_OperatingIncomeLoss	21,110us-gaap_OperatingIncomeLoss	21,858us-gaap_OperatingIncomeLoss	99,999us-gaap_OperatingIncomeLoss	99,663us-gaap_OperatingIncomeLoss	96,221us-gaap_OperatingIncomeLoss
Net income	11,112	23,892	11,194	20,905	15,162	23,996	11,658	12,630	67,103	63,445	55,681
Basic and Diluted Earnings Per Share (in dollars per share)	$ 0.26	$ 0.55	$ 0.26	$ 0.48	$ 0.35	$ 0.56	$ 0.27	$ 0.30	$ 1.55	$ 1.48	$ 1.32
SPP Integrated Market [Member]
Operating revenues	$ 7,500	$ 11,500	$ 16,500	$ 6,200

The amount of net income (loss) from continuing operations per each basic and diluted share of common stock or unit when the per share amount is the same for both basic and diluted shares.

No definition available.

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- Definition

Net income after adjustments for dividends on preferred stock (declared in the period) and/or cumulative preferred stock (accumulated for the period).

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 260

-SubTopic 10

-Section 45

-Paragraph 11

-URI http://asc.fasb.org/extlink&oid=7655603&loc=d3e1377-109256

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- Definition

The net result for the period of deducting operating expenses from operating revenues.

+ References

No definition available.

+ Details

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- Definition

The total amount of regulated operating revenues recognized during the period.

+ References

No definition available.

+ Details

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