DEBT, CREDIT FACILITY, AND CAPITAL LEASE OBLIGATIONS (Capital Lease) (Details) $ in Thousands	1 Months Ended			12 Months Ended
	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Apr. 30, 2015 USD ($)	Dec. 31, 2017 USD ($) lease	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Sep. 16, 2016	Jan. 31, 2016 MW	May 31, 2015 USD ($)	Jan. 31, 2015 USD ($) MW
Debt Instrument [Line Items]
Capital Lease Obligations	$ 91,000			$ 39,000	$ 91,000
Less Current Obligations Under Capital Leases	51,765			10,749	51,765
Total Capital Lease Obligations, Non-Current	39,267			$ 28,519	39,267
Lessee, Operating Lease, Renewal Term				2 years
Payments to Acquire Equipment on Lease				$ 0	0	$ 120,312
Fixed purchase price				0	$ 36,000	$ 0
Interest Rate Swap
Debt Instrument [Line Items]
Notional amount				$ 18,000
Springerville Common Facilities Lease Debt | Interest Rate Swap
Debt Instrument [Line Items]
Fixed rate of interest related to interest rate swap				5.77%
Notional amount				$ 18,000
Notional amount of debt	$ 3,000
Derivative basis spread	1.88%			1.88%	1.88%
Springerville Common Facility Lease Expiring January 2021
Debt Instrument [Line Items]
Number of operating lease | lease				2
Percentage of interest committed to purchase				14.00%
Springerville Common Facilities
Debt Instrument [Line Items]
Jointly Owned, Percentage of Interest Purchased				17.80%
Fixed purchase price				$ 68,000
Springerville Common Facilities | Springerville Common Facility Lease Expiring December 2017
Debt Instrument [Line Items]
Increase (Decrease) in Capital Lease Obligation				36,000
Payments to Acquire Equipment on Lease				$ 38,000
Springerville Unit One
Debt Instrument [Line Items]
Percentage of ownership in generating units (in percentage)	100.00%			32.20%	100.00%
Springerville Unit One | Additional Purchase of Equity Interest
Debt Instrument [Line Items]
Percentage of ownership in generating station										24.80%
Generating capacity purchased, in MWs | MW										96
Springerville Unit One | Completion of Purchase of Equity Interest
Debt Instrument [Line Items]
Percentage of ownership in generating station										49.50%
Generating capacity purchased, in MWs | MW								387		192
Percentage of ownership in generating units (in percentage)							100.00%
Fixed price to acquire leased interest in facilities		$ 85,000
Springerville Unit One | Springerville Unit One Lease | Additional Purchase of Equity Interest
Debt Instrument [Line Items]
Lease arrangement, fair market value purchase price										$ 46,000
Springerville Coal Handling Facilities
Debt Instrument [Line Items]
Percentage of ownership in generating station				83.00%
Springerville Coal Handling Facilities | SRP
Debt Instrument [Line Items]
Percentage of ownership in generating station									17.05%
Springerville Coal Handling Facilities | Additional Purchase of Equity Interest
Debt Instrument [Line Items]
Percentage of ownership in generating station			86.70%	67.80%
Springerville Coal Handling Facilities | Completion of Purchase of Equity Interest
Debt Instrument [Line Items]
Percentage of ownership in generating units (in percentage)			100.00%
Springerville Coal Handling Facilities | Springerville Coal Handling Facilities Lease | SRP
Debt Instrument [Line Items]
Sales price of leased interest in facilities									$ 24,000
Springerville Coal Handling Facilities | Springerville Coal Handling Facilities Lease | Additional Purchase of Equity Interest
Debt Instrument [Line Items]
Fixed price to acquire leased interest in facilities			$ 120,000