SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details 3) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Depreciable plant and equipment
Equipment	$ 2,729,757	$ 2,522,174
Transmission and distribution plant	$ 22,212	$ 21,925
Office Buildings and other structures, Useful Lives	50 years
Provision for depreciation expressed as a percentage of the aggregate depreciable asset balances	3.00%	2.70%	2.80%
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	$ 592,612	$ 561,909
Office buildings and other structures	245,877	218,711
Transmission and distribution plant	1,891,268	1,741,554
Total	$ 2,729,757	$ 2,522,174