UTILITY PLANT AND JOINTLY-OWNED FACILITIES (Utility Plant in Service by Major Class) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Public Utility, Property, Plant and Equipment [Line Items]
Annual Depreciation Rate	3.08%	3.13%	2.97%
Plant in Service	$ 6,663,912	$ 6,020,469
Capitalized computer software, net	78,000	73,000
Amortization of computer software costs	$ 26,000	24,000	$ 19,000
Acquired finite-lived intangible assets, weighted average useful life (in years)	3 years
Plant acquisition adjustments	$ (211,000)	(134,000)
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
Generation Plant
Public Utility, Property, Plant and Equipment [Line Items]
Annual Depreciation Rate	3.19%
Average Remaining Life in Years	20 years
Plant in Service	$ 3,065,000	2,667,000
Transmission Plant
Public Utility, Property, Plant and Equipment [Line Items]
Annual Depreciation Rate	1.69%
Average Remaining Life in Years	37 years
Plant in Service	$ 1,060,000	1,010,000
Distribution Plant
Public Utility, Property, Plant and Equipment [Line Items]
Annual Depreciation Rate	1.56%
Average Remaining Life in Years	31 years
Plant in Service	$ 1,784,000	1,692,000
General Plant
Public Utility, Property, Plant and Equipment [Line Items]
Annual Depreciation Rate	5.89%
Average Remaining Life in Years	20 years
Plant in Service	$ 477,000	409,000
Intangible Plant, Software Costs and Other
Public Utility, Property, Plant and Equipment [Line Items]
Plant in Service	271,000	239,000
Plant Held for Future Use
Public Utility, Property, Plant and Equipment [Line Items]
Plant in Service	$ 7,000	$ 3,000