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, 2017	Dec. 31, 2016	Dec. 31, 2015
Public Utility, Property, Plant and Equipment [Line Items]
Capital Leases	$ 2,554	$ 3,356	$ 3,994
Total Lease Expense	9,000	8,000	12,000
Interest Expense, Capital Leases
Public Utility, Property, Plant and Equipment [Line Items]
Capital Leases	3,000	3,000	4,000
Operating Expenses, Fuel
Public Utility, Property, Plant and Equipment [Line Items]
Amortization of Capital Lease Assets	0	0	2,000
Operating Expenses, Amortization
Public Utility, Property, Plant and Equipment [Line Items]
Amortization of Capital Lease Assets	$ 6,000	$ 5,000	$ 6,000