SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details 4) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Depreciable plant and equipment
Equipment	$ 2,043,414	$ 1,903,365
Transmission and distribution plant	18,232	18,613
Provision for depreciation expressed as a percentage of the aggregate depreciable asset balances	3.03%	3.10%	3.00%
Asset Retirement Obligation
Estimated retirement obligation	17,036	16,105
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
Office Buildings and other structures, Useful Lives	50 years
Utility Plant
Depreciable plant and equipment
Equipment	443,439	400,610
Office buildings and other structures	179,023	151,597
Transmission and distribution plant	1,420,952	1,351,158
Total	$ 2,043,414	$ 1,903,365