REGULATORY MATTERS (Regulatory Assets and liabilities- Footnotes) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Regulatory Assets [Line Items]
Remaining Recovery Period (years)	1 year
Accumulated depreciation and amortization	$ 2,193,656	$ 2,385,053
Property Tax Deferrals
Regulatory Assets [Line Items]
Remaining Recovery Period (years)	1 year
Springerville Unit 1 Leasehold Improvements
Regulatory Assets [Line Items]
Remaining Recovery Period (years)	6 years
Amortization period authorized to recover leasehold improvement costs at Springerville Unit 1	10 years
Net Cost of Removal
Regulatory Assets [Line Items]
Accumulated depreciation and amortization	$ 87,000