REGULATORY MATTERS (Regulatory Assets) (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 264,000	$ 292,000
Less Current Portion	51,841	69,383
Total Non-Current Regulatory Assets	212,312	223,192
Pension and Other Postretirement Plans Costs [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	120,000	126,000
Final Mine Reclamation and Retiree Health Care Costs [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	28,000	29,000
Deferred Income Tax Charge [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	26,000	31,000
Property Tax Deferrals [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	21,000	21,000
Springerville Unit 1 Leasehold Improvements - Third Party Owners [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	21,000	0
Lost Fixed Cost Recovery and Demand Side Management [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	16,000	12,000
Deferred Derivative Gain (Loss) [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	12,000	18,000
Purchased Power and Fuel Adjustment Clause [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	0	19,000
Springerville Deferrals [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	0	16,000
Other Regulatory Assets (Liabilities) [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 20,000	$ 20,000