UTILITY PLANT AND JOINTLY-OWNED FACILITES (Utility Plant in Service by Major Class) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Public Utility, Property, Plant and Equipment [Line Items]
Annual Depreciation Rate	2.97%	2.85%	2.83%
Plant in Service	$ 5,780,805	$ 5,975,139
Utility Plant under Capital Leases	84,870	167,413
Plant acquisition adjustments	$ (134,000)	(139,000)
Generation Plant
Public Utility, Property, Plant and Equipment [Line Items]
Annual Depreciation Rate	3.19%
Average Remaining Life in Years	25 years
Plant in Service	$ 2,548,000	2,866,000
Transmission Plant
Public Utility, Property, Plant and Equipment [Line Items]
Annual Depreciation Rate	1.48%
Average Remaining Life in Years	32 years
Plant in Service	$ 1,001,000	1,024,000
Distribution Plant
Public Utility, Property, Plant and Equipment [Line Items]
Annual Depreciation Rate	1.56%
Average Remaining Life in Years	36 years
Plant in Service	$ 1,632,000	1,512,000
General Plant
Public Utility, Property, Plant and Equipment [Line Items]
Annual Depreciation Rate	5.89%
Average Remaining Life in Years	12 years
Plant in Service	$ 389,000	381,000
Intangible Plant, Software Costs and Other
Public Utility, Property, Plant and Equipment [Line Items]
Plant in Service	207,000	185,000
Capitalized computer software, net	59,000	52,000
Amortization of computer software costs	19,000	17,000	$ 14,000
Plant Held for Future Use
Public Utility, Property, Plant and Equipment [Line Items]
Plant in Service	$ 4,000	$ 7,000
Enterprise Software
Public Utility, Property, Plant and Equipment [Line Items]
Average Remaining Life in Years	3 years
Minimum | Application Software
Public Utility, Property, Plant and Equipment [Line Items]
Acquired finite-lived intangible assets, weighted average useful life (in years)	3 years
Maximum | Application Software
Public Utility, Property, Plant and Equipment [Line Items]
Acquired finite-lived intangible assets, weighted average useful life (in years)	5 years