SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details 4) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Depreciable plant and equipment
Total	$ 1,903,365	$ 1,807,569
Provision for depreciation expressed as a percentage of the aggregate depreciable asset balances	3.10%	3.00%	2.80%
Asset Retirement Obligation
Estimated retirement obligation	16,105	14,049
Minimum
Depreciable plant and equipment
Equipment, Useful Lives (in years)	P5Y
Transmission and distribution plant, Useful Lives (in years)	P40Y
Maximum
Depreciable plant and equipment
Equipment, Useful Lives (in years)	P50Y
Transmission and distribution plant, Useful Lives (in years)	P65Y
Office Buildings and other structures, Useful Lives (in years)	P50Y
Utility Plant
Depreciable plant and equipment
Equipment	400,610	382,195
Transmission and distribution plant	1,351,158	1,287,010
Office buildings and other structures	151,597	138,364
Total	$ 1,903,365	$ 1,807,569