UTILITY PLANT AND JOINTLY-OWNED FACILITES (Utility Plant in Service by Major Class) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Public Utility, Property, Plant and Equipment [Line Items]
Annual Depreciation Rate	3.13%	2.97%	2.85%
Plant in Service	$ 6,020,469	$ 5,780,805
Utility Plant under Capital Leases	248,635	84,870
Capitalized computer software, net	73,000	59,000
Amortization of computer software costs	$ 24,000	19,000	$ 17,000
Acquired finite-lived intangible assets, weighted average useful life (in years)	3 years
Plant acquisition adjustments	$ (134,000)	(134,000)
Generation Plant
Public Utility, Property, Plant and Equipment [Line Items]
Annual Depreciation Rate	3.19%
Average Remaining Life in Years	24 years
Plant in Service	$ 2,667,000	2,548,000
Transmission Plant
Public Utility, Property, Plant and Equipment [Line Items]
Annual Depreciation Rate	1.48%
Average Remaining Life in Years	31 years
Plant in Service	$ 1,010,000	1,001,000
Distribution Plant
Public Utility, Property, Plant and Equipment [Line Items]
Annual Depreciation Rate	1.56%
Average Remaining Life in Years	35 years
Plant in Service	$ 1,692,000	1,632,000
General Plant
Public Utility, Property, Plant and Equipment [Line Items]
Annual Depreciation Rate	5.89%
Average Remaining Life in Years	11 years
Plant in Service	$ 409,000	389,000
Intangible Plant, Software Costs and Other
Public Utility, Property, Plant and Equipment [Line Items]
Plant in Service	239,000	207,000
Plant Held for Future Use
Public Utility, Property, Plant and Equipment [Line Items]
Plant in Service	$ 3,000	$ 4,000
Minimum
Public Utility, Property, Plant and Equipment [Line Items]
Acquired finite-lived intangible assets, weighted average useful life (in years)	3 years
Maximum
Public Utility, Property, Plant and Equipment [Line Items]
Acquired finite-lived intangible assets, weighted average useful life (in years)	5 years