UTILITY PLANT AND JOINTLY-OWNED FACILITES (Amount of Lease Expense Incurred for Generation-Related Capital Leases) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Public Utility, Property, Plant and Equipment [Line Items]
Capital Leases		$ 0	$ 0
Total Lease Expense	$ 16,000	9,000	8,000
Interest Expense, Capital Leases
Public Utility, Property, Plant and Equipment [Line Items]
Capital Leases	2,000	3,000	3,000
Operating Expenses, Fuel
Public Utility, Property, Plant and Equipment [Line Items]
Capital Leases	8,000	0	0
Operating Expenses, Amortization
Public Utility, Property, Plant and Equipment [Line Items]
Amortization of Capital Lease Assets	$ 6,000	$ 6,000	$ 5,000
Springerville Unit One
Public Utility, Property, Plant and Equipment [Line Items]
Percentage of ownership in generating units (in percentage)	32.20%