VARIABLE INTEREST ENTITIES - Consolidated Entities (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Consolidated Variable Interest Entities
Net electric plant	$ 3,355,091		$ 3,613,846
Noncontrolling interest	111,768		111,717
Long-term debt	3,217,571		3,063,351
Accrued interest	47,037		29,716
Interest	$ 36,906	$ 38,281
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%
Springerville Partnership \| Reportable Legal Entities
Consolidated Variable Interest Entities
Net electric plant	$ 771,876		776,411
Noncontrolling interest	111,768		111,717
Long-term debt	343,313		380,867
Accrued interest	3,977		$ 11,050
Depreciation and amortization	4,534	4,534
Interest	$ 5,861	$ 6,831
Other Limited Partners \| Springerville Partnership
Consolidated Variable Interest Entities
Ownership interest held by noncontrolling interest (as a percent)	49.00%

Amount of the cost of borrowed funds, net of capitalized interest costs, accounted for as interest expense

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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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The aggregate expense recognized in the current period that allocates the cost of tangible assets, intangible assets, or depleting assets to periods that benefit from use of the assets.

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Carrying value as of the balance sheet date of [accrued] interest payable on all forms of debt, including trade payables, that has been incurred and is unpaid. Used to reflect the current portion of the liabilities (due within one year or within the normal operating cycle if longer).

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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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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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