DEBT, CREDIT FACILITY, AND CAPITAL LEASE OBLIGATIONS (Capital Lease) (Details) $ in Thousands		1 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018 USD ($)	May 30, 2018 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2017 MW	Dec. 31, 2019 USD ($)
Long-term Contract for Purchase of Electric Power [Line Items]
Gila river unit 2, capital lease					$ 164,000	$ 0
Interest expense			$ 16,626	$ 16,291	49,818	$ 48,972
Minimum Lease Payments
2018	$ 3,000		3,000		3,000
2019	176,000		176,000		176,000
2020	2,000		2,000		2,000
2021	0		0		0
2022	0		0		0
Total 2018-2022	181,000		181,000		181,000
Thereafter	0		0		0
Less: Imputed Interest	(17,000)		(17,000)		(17,000)
Total	164,000		164,000		164,000
Salt River Project Agricultural Improvement and Power District
Long-term Contract for Purchase of Electric Power [Line Items]
PPA term (years)							20 years
Capacity purchased (power) | MW							550
Call option to purchase the unit, term (years)							3 years
Gila river unit 2, capital lease	164,000
Capital lease obligation, term (months)		20 months
Estimated monthly charge		$ 1,000
Interest expense			3,000		5,000
Other Assets | Salt River Project Agricultural Improvement and Power District
Long-term Contract for Purchase of Electric Power [Line Items]
Amortization	$ 2,000		$ 2,000		$ 2,000
Scenario, Forecast | Salt River Project Agricultural Improvement and Power District
Long-term Contract for Purchase of Electric Power [Line Items]
Estimated purchase price								$ 164,000