SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details 3) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Depreciable plant and equipment
Equipment	$ 2,950,424	$ 2,729,757
Transmission and distribution plant	$ 24,743	$ 22,212
Office Buildings and other structures, Useful Lives	50 years
Provision for depreciation expressed as a percentage of the aggregate depreciable asset balances	3.02%	3.00%	2.70%
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	$ 643,581	$ 592,612
Office buildings and other structures	267,948	245,877
Transmission and distribution plant	2,038,895	1,891,268
Total	$ 2,950,424	$ 2,729,757