VARIABLE INTEREST ENTITIES - Consolidated Entities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Consolidated Variable Interest Entities
Noncontrolling interest included in consolidated statements of financial position (in dollars)	$ 111,295	$ 109,147
Interest expense (in dollars)	$ 147,608	144,877	$ 142,570
Primary Beneficiary | Springerville Partnership
Consolidated Variable Interest Entities
Equity interest (as a percent)	51.00%
General partner interest (as a percent)	1.00%
Ownership interest held by noncontrolling interest (as a percent)	49.00%
Noncontrolling interest included in consolidated statements of financial position (in dollars)	$ 111,300	109,100
Springerville Partnership | Primary Beneficiary
Consolidated Variable Interest Entities
Depreciation and amortization (in dollars)	19,600	21,000	21,000
Interest expense (in dollars)	28,400	30,400	$ 32,300
Springerville Partnership | Primary Beneficiary | Electric Plant
Consolidated Variable Interest Entities
Assets of variable interest entity included in consolidated statements of financial position (in dollars)	812,700	832,300
Springerville Partnership | Primary Beneficiary | Long-term debt
Consolidated Variable Interest Entities
Liabilities of variable interest entity included in consolidated statements of financial position (in dollars)	431,300	472,100
Springerville Partnership | Primary Beneficiary | Accrued interest included in long-term debt
Consolidated Variable Interest Entities
Liabilities of variable interest entity included in consolidated statements of financial position (in dollars)	$ 12,400	$ 13,400
Springerville Partnership | Primary Beneficiary | Springerville Unit 3 Lease
Consolidated Variable Interest Entities
Percentage of financial and other support provided	100.00%
Springerville Partnership | Springerville Unit 3 Holding LLC
Consolidated Variable Interest Entities
Ownership interest held by parent (as a percent)	100.00%