RELATED PARTY TRANSACTIONS (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Related Party Transaction, Due from (to) Related Parties [Abstract]
Receivables from Related Parties	$ 7,000	$ 9,000
Payables to Related Parties	4,000	2,000
Contribution from Parent	0	0	$ 180,000
Dividends Declared to Parent	(70,000)	(50,000)	(50,000)
Transmission Revenues, UNS Electric
Related Party Transaction, Due from (to) Related Parties [Abstract]
Transmission and Wholesale Revenue	7,000	7,000	6,000
Wholesale Revenues, UNS Electric
Related Party Transaction, Due from (to) Related Parties [Abstract]
Transmission and Wholesale Revenue	0	0	2,000
Control Area Services, UNS Electric
Related Party Transaction, Due from (to) Related Parties [Abstract]
Control Area Services, UNS Electric	3,000	2,000	2,000
Common Costs, UNS Energy Affiliates
Related Party Transaction, Due from (to) Related Parties [Abstract]
Common Costs, UNS Energy Affiliates	16,000	14,000	12,000
Corporate Services, Fortis Affiliates
Related Party Transaction, Due from (to) Related Parties [Abstract]
Transmission and Wholesale Revenue	2,000	0	0
Wholesale Revenues, UNS Electric
Related Party Transaction, Due from (to) Related Parties [Abstract]
Wholesale Revenues, UNS Electric	0	1,000	1,000
Supplemental Workforce, SES
Related Party Transaction, Due from (to) Related Parties [Abstract]
Supplemental Workforce, SES	15,000	14,000	16,000
Corporate Services, UNS Energy
Related Party Transaction, Due from (to) Related Parties [Abstract]
Corporate Services	$ 5,000	7,000	7,000
Intercompany allocation parent to subsidiary (in percentage)	82.00%
Management fee	$ 6,000	6,000	5,000
Corporate Services, UNS Energy Affiliates
Related Party Transaction, Due from (to) Related Parties [Abstract]
Corporate Services	5,000	4,000	$ 1,000
UNS Electric
Related Party Transaction, Due from (to) Related Parties [Abstract]
Receivables from Related Parties	5,000	7,000
UNS Gas
Related Party Transaction, Due from (to) Related Parties [Abstract]
Receivables from Related Parties	2,000	2,000
SES
Related Party Transaction, Due from (to) Related Parties [Abstract]
Payables to Related Parties	3,000	2,000
UNS Energy
Related Party Transaction, Due from (to) Related Parties [Abstract]
Payables to Related Parties	$ 1,000	$ 0

Common Costs Charged to Related Parties - Common Cost Allocations

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Intercompany Allocation Parent to Subsidiary

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management fee

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Amount for accounts payable to related parties. 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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Amount of receivables arising from transactions with related parties due within one year or the normal operating cycle, if longer.

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Maintenance costs incurred and directly related to services rendered by an entity during the reporting period.

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Amount of paid and unpaid common stock dividends declared with the form of settlement in cash, stock and payment-in-kind (PIK).

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The cash inflow from parent as a source of financing that is recorded as additional paid in capital.

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Reflects the sum of all other revenue and income realized from sales and other transactions (excluding transactions that are eliminated in consolidated or combined financial statements) with related party during the period.

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Purchases during the period (excluding transactions that are eliminated in consolidated or combined financial statements) with related party.

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Amount of selling, general and administrative expenses resulting from transactions, excluding transactions that are eliminated in consolidated or combined financial statements, with related party.

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Amount of revenue, fees and commissions earned from transactions between (a) a parent company and its subsidiaries; (b) subsidiaries of a common parent; (c) an entity and trusts for the benefit of employees, for example, but not limited to, pension and profit-sharing trusts that are managed by or under the trusteeship of the entity's management; (d) an entity and its principal, owners, management, or members of their immediate families; and (e) affiliates.

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