SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details 4) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Depreciable plant and equipment
Equipment	$ 2,191,194	$ 2,043,414
Transmission and distribution plant	18,768	18,232
Office Buildings and other structures, Useful Lives	50 years
Provision for depreciation expressed as a percentage of the aggregate depreciable asset balances	2.97%	3.03%	3.10%
Schedule of Retirement obligation
Beginning balance	17,036us-gaap_AssetRetirementObligation
Ending balance	18,414us-gaap_AssetRetirementObligation	17,036us-gaap_AssetRetirementObligation
Minimum
Depreciable plant and equipment
Equipment, Useful Lives	5 years
Transmission and distribution plant, Useful Lives	40 years
Maximum
Depreciable plant and equipment
Equipment, Useful Lives	50 years
Transmission and distribution plant, Useful Lives	65 years
Utility Plant
Depreciable plant and equipment
Equipment	487,845	443,439
Office buildings and other structures	194,905	179,023
Transmission and distribution plant	1,508,444	1,420,952
Total	$ 2,191,194	$ 2,043,414

Period end amount of property, plant and equipment and Office buildings and other structures owned by public utility.

No definition available.

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- Definition

The carrying amount of a liability for an asset retirement obligation. An asset retirement obligation is a legal obligation associated with the disposal or retirement of a tangible long-lived asset that results from the acquisition, construction or development, or the normal operations of a long-lived asset, except for certain obligations of lessees.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 410

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Asset Retirement Obligation

-URI http://asc.fasb.org/extlink&oid=6505190

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- Definition

Useful life of assets common to business units owned by public utility, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

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- Definition

Composite depreciation rate for public utility plants in service.

+ References

No definition available.

+ Details

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- Definition

Period end book value of equipment owned (but not classified elsewhere) by the public utility.

+ References

No definition available.

+ Details

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- Definition

Useful life of other equipment owned by public utility, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

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- Definition

Period end book value of other property, plant and equipment (PPE) owned (but not classified elsewhere) by the public utility.

+ References

No definition available.

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- Definition

Period end amount of property, plant and equipment (PPE) related to transmission and distribution owned by public utility.

+ References

No definition available.

+ Details

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- Definition

Useful life of utility transmission equipment owned by public utility, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

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