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Long-term Contract for Purchase of Electric Power, Option to Purchase the Unit, Term

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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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Term of lessee's operating lease renewal, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

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Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
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Term of lessee's operating lease, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

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Amount of lease payments to be received by lessor in fifth fiscal year following latest fiscal year for operating lease. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest statement of financial position date.

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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Amount of lease payments to be received by lessor in fourth year following latest fiscal year for operating lease. Excludes interim and annual periods when interim periods are reported on rolling approach, from latest statement of financial position date.

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Amount of lease payments to be received by lessor in next fiscal year following latest fiscal year for operating lease. Excludes interim and annual periods when interim periods are reported on rolling approach, from latest statement of financial position date.

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Amount of lease payments to be received by lessor after fifth fiscal year following latest statement of financial position date for operating lease. Excludes interim and annual periods when interim periods are reported on rolling approach, from latest statement of financial position date.

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Amount of lease payments to be received by lessor in third fiscal year following latest fiscal year for operating lease. Excludes interim and annual periods when interim periods are reported on rolling approach, from latest statement of financial position date.

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Amount of lease payments to be received by lessor in second fiscal year following latest fiscal year for operating lease. Excludes interim and annual periods when interim periods are reported on rolling approach, from latest statement of financial position date.

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Amount of single lease cost, calculated by allocation of remaining cost of lease over remaining lease term. Includes, but is not limited to, single lease cost, after impairment of right-of-use asset, calculated by amortization of remaining right-of-use asset and accretion of lease liability.

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-URI http://asc.fasb.org/extlink&oid=77888399&loc=SL77918982-209971

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Amount of operating lease income from lease payments and variable lease payments paid and payable to lessor. Includes, but is not limited to, variable lease payments not included in measurement of lease receivable.

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LEASES (Narrative) (Details) $ in Millions	12 Months Ended
LEASES (Narrative) (Details) $ in Millions	Dec. 31, 2019 USD ($) option	Dec. 31, 2018 USD ($)	Dec. 31, 2017 MWh	Jan. 31, 2021	May 31, 2018
Lessee, Lease, Description [Line Items]
Renewal term	15 years
Number of renewal options \| option	1
Operating lease income	$ 1
Operating lease cost	1	$ 1
Lessor, Operating Lease, Payments, Fiscal Year Maturity
2020	1
2021	1
2022	1
2023	1
2024	1
Thereafter	$ 1
Tolling PPA
Lessee, Lease, Description [Line Items]
Term of contract					20 years
Generating capacity (MW) \| MWh			550
Option to purchase unit, term			3 years
Energy Storage
Lessee, Lease, Description [Line Items]
Lease not yet commenced, term of contract	20 years
Office Facility and Utility Property
Lessee, Lease, Description [Line Items]
Remaining lease term	3 years
Springerville Common Facilities
Lessee, Lease, Description [Line Items]
Commitment to purchase finance lease interest	$ 68
Springerville Common Facilities \| Forecast
Lessee, Lease, Description [Line Items]
Percentage of interest to be purchased (in percentage)				32.20%
Springerville Common Facilities \| Tri-State
Lessee, Lease, Description [Line Items]
Percentage of interest to be purchased (in percentage)	14.00%
Springerville Common Facilities \| SRP
Lessee, Lease, Description [Line Items]
Percentage of interest to be purchased (in percentage)	14.00%
Minimum
Lessee, Lease, Description [Line Items]
Remaining lease term	1 year
Minimum \| Office Facility and Utility Property
Lessee, Lease, Description [Line Items]
Renewal term	4 years
Maximum
Lessee, Lease, Description [Line Items]
Remaining lease term	22 years
Maximum \| Office Facility and Utility Property
Lessee, Lease, Description [Line Items]
Remaining lease term	13 years