Represents federal and state income tax expense (benefit).

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Represents the cost of borrowed funds accounted for as interest that was charged against earnings during the period, which excludes the component of the allowance for funds used for construction during the period that comprise interest on borrowed funds used in financing the construction of regulated assets.

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Represents the number of towns to which the entity provides water service.

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Sum of the carrying amounts as of the balance sheet date of all assets that are recognized. Assets are probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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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.

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This element represents the income or loss from continuing operations attributable to the economic entity which may also be defined as revenue less expenses and taxes from ongoing operations before extraordinary items, and noncontrolling interest.

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Amount before accretion (amortization) of purchase discount (premium) of interest income on nonoperating securities.

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

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Number of segments reported by the entity. A reportable segment is a component of an entity for which there is an accounting requirement to report separate financial information on that component in the entity's financial statements.

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

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The cash outflow for purchases of and capital improvements on property, plant and equipment (capital expenditures), software, and other intangible assets.

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Total increase in earnings in the period representing the cost of equity (based on assumed rate of return) and/or borrowed funds (based on interest rate) used to finance construction of regulated assets, which is expected to be recovered through rate adjustments.

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

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Amount of operating expense for depreciation and amortization of regulated operation.

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SEGMENT INFORMATION (Details) (USD $) In Thousands, 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 segment item	Dec. 31, 2013	Dec. 31, 2012
Segment Information
Number of business segments									3us-gaap_NumberOfReportableSegments
Number of towns to which water service is provided									3
Statement of Income Information, balance sheet information and capital expenditures of business segments
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
Depreciation and amortization									73,185	69,306	60,447
Federal and state income taxes									39,220	37,492	34,159
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
Interest income									51us-gaap_InvestmentIncomeInterest	566us-gaap_InvestmentIncomeInterest	972us-gaap_InvestmentIncomeInterest
Interest expense									41,740	41,479	41,467
Income from AFUDC (debt and equity)									9,917	5,940	1,928
Net income	11,112	23,892	11,194	20,905	15,162	23,996	11,658	12,630	67,103	63,445	55,681
Capital Expenditures									222,853	160,208	146,287
Total assets	2,390,256us-gaap_Assets				2,145,045us-gaap_Assets				2,390,256us-gaap_Assets	2,145,045us-gaap_Assets
Goodwill	39,492us-gaap_Goodwill				39,492us-gaap_Goodwill				39,492us-gaap_Goodwill	39,492us-gaap_Goodwill
Income from continuing operations									67,103	63,445	55,681
SPP Integrated Market [Member]
Statement of Income Information, balance sheet information and capital expenditures of business segments
Revenues	7,500	11,500	16,500	6,200
Electric
Statement of Income Information, balance sheet information and capital expenditures of business segments
Revenues									592,491	536,413	510,653
Depreciation and amortization									67,534	63,659	55,312
Federal and state income taxes									35,737	34,478	32,266
Operating income									90,488	90,984	89,445
Interest income									37	537	946
Interest expense									37,911	37,683	37,866
Income from AFUDC (debt and equity)									9,833	5,910	1,918
Capital Expenditures									212,866	153,401	140,117
Total assets	2,271,539				2,034,234				2,271,539	2,034,234
Income from continuing operations									61,467	58,603	52,631
Electric \| SPP Integrated Market [Member]
Statement of Income Information, balance sheet information and capital expenditures of business segments
Revenues									41,900
Gas
Statement of Income Information, balance sheet information and capital expenditures of business segments
Revenues									51,842	50,041	39,849
Depreciation and amortization									3,760	3,709	3,598
Federal and state income taxes									1,840	1,484	789
Operating income									6,775	6,194	5,005
Interest income									25	115	323
Interest expense									3,861	3,890	3,905
Income from AFUDC (debt and equity)									84	30	10
Capital Expenditures									7,836	4,419	3,571
Total assets	130,856				123,736				130,856	123,736
Goodwill	39,492				39,492				39,492	39,492
Income from continuing operations									2,965	2,355	1,256
Others
Statement of Income Information, balance sheet information and capital expenditures of business segments
Revenues									9,302	9,147	7,187
Depreciation and amortization									1,891	1,938	1,537
Federal and state income taxes									1,643	1,530	1,104
Operating income									2,736	2,485	1,771
Interest income									21	8	7
Capital Expenditures									2,151	2,388	2,599
Total assets	34,655				31,306				34,655	31,306
Income from continuing operations									2,671	2,487	1,794
Eliminations
Statement of Income Information, balance sheet information and capital expenditures of business segments
Revenues									(1,305)	(1,271)	(592)
Interest income									(32)	(94)	(304)
Interest expense									(32)	(94)	(304)
Total assets	$ (46,794)				$ (44,231)				$ (46,794)	$ (44,231)