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The percentage of financial or other support the reporting entity has provided (explicitly or implicitly) to the Variable Interest Entity (VIE).

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Percentage investment held by the managing member or general partner of the limited liability company (LLC) or limited partnership (LP).

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Amount after unamortized (discount) premium and debt issuance costs of long-term debt classified as noncurrent and excluding amounts to be repaid within one year or the normal operating cycle, if longer. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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Total of all stockholders' equity (deficit) items, net of receivables from officers, directors, owners, and affiliates of the entity which is directly or indirectly attributable to that ownership interest in subsidiary equity which is not attributable to the parent (that is, noncontrolling interest, previously referred to as minority interest).

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The equity interest of noncontrolling shareholders, partners or other equity holders in consolidated entity.

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The parent entity's interest in net assets of the subsidiary, expressed as a percentage.

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Period end amount of total net PPE.

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VARIABLE INTEREST ENTITIES - Consolidated Entities (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Consolidated Variable Interest Entities
Net electric plant	$ 3,339,573		$ 3,339,573		$ 3,613,846
Noncontrolling interest	113,425		113,425		111,717
Long-term debt	3,205,904		3,205,904		3,063,351
Accrued interest	45,830		45,830		29,716
Interest	$ 36,213	$ 38,030	$ 109,511	$ 113,954
Springerville Partnership
Consolidated Variable Interest Entities
Equity interest (as a percent)			51.00%
General partner interest (as a percent)			1.00%
Springerville Partnership \| 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%		100.00%
Springerville Partnership \| Reportable Legal Entities
Consolidated Variable Interest Entities
Net electric plant	$ 762,808		$ 762,808		776,411
Noncontrolling interest	113,425		113,425		111,717
Long-term debt	342,680		342,680		380,867
Accrued interest	3,977		3,977		$ 11,050
Depreciation and amortization	4,535	4,535	13,603	13,603
Interest	$ 5,646	$ 6,281	$ 17,157	$ 19,045
Other Limited Partners \| Springerville Partnership
Consolidated Variable Interest Entities
Ownership interest held by noncontrolling interest (as a percent)	49.00%		49.00%