RELATED PARTY TRANSACTIONS (Details) $ in Thousands	12 Months Ended
Dec. 31, 2018 USD ($) facility	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Related Party Transaction, Due from (to) Related Parties [Abstract]
Receivables from Related Parties	$ 8,000	$ 7,000
Payables to Related Parties	5,000	4,000
Supplemental Workforce, SES	543,143	425,555	$ 420,061
Transmission Revenues, UNS Electric
Related Party Transaction, Due from (to) Related Parties [Abstract]
Transmission and Wholesale Revenue	6,000	7,000	7,000
Wholesale Revenues, UNS Electric
Related Party Transaction, Due from (to) Related Parties [Abstract]
Transmission and Wholesale Revenue	1,000	0	0
Control Area Services, UNS Electric
Related Party Transaction, Due from (to) Related Parties [Abstract]
Control Area Services and Corporate Services	3,000	3,000	2,000
Common Costs, UNS Energy Affiliates
Related Party Transaction, Due from (to) Related Parties [Abstract]
Common Costs, UNS Energy Affiliates	18,000	16,000	14,000
Corporate Services, Fortis Affiliates
Related Party Transaction, Due from (to) Related Parties [Abstract]
Control Area Services and Corporate Services	0	2,000	0
Wholesale Revenues, UNS Electric
Related Party Transaction, Due from (to) Related Parties [Abstract]
Wholesale Revenues, UNS Electric	0	0	1,000
Supplemental Workforce, SES
Related Party Transaction, Due from (to) Related Parties [Abstract]
Supplemental Workforce, SES	15,000	15,000	14,000
Corporate Services, UNS Energy
Related Party Transaction, Due from (to) Related Parties [Abstract]
Corporate Services	$ 6,000	5,000	7,000
Intercompany allocation parent to subsidiary (in percentage)	82.00%
Management fee	$ 5,000	6,000	6,000
Corporate Services, UNS Energy Affiliates
Related Party Transaction, Due from (to) Related Parties [Abstract]
Corporate Services	7,000	5,000	4,000
Capacity Charges, UNS Gas
Related Party Transaction, Due from (to) Related Parties [Abstract]
Capacity Charges, UNS Gas	$ 1,000	0	$ 0
Number of generation facilities \| facility	1
SES
Related Party Transaction, Due from (to) Related Parties [Abstract]
Payables to Related Parties	$ 2,000	3,000
UNS Electric
Related Party Transaction, Due from (to) Related Parties [Abstract]
Receivables from Related Parties	7,000	5,000
Payables to Related Parties	1,000	0
UNS Gas
Related Party Transaction, Due from (to) Related Parties [Abstract]
Receivables from Related Parties	1,000	2,000
Payables to Related Parties	1,000	0
UNS Energy
Related Party Transaction, Due from (to) Related Parties [Abstract]
Payables to Related Parties	$ 1,000	$ 1,000

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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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The aggregate costs related to goods produced and sold and services rendered by an entity during the reporting period. This excludes costs incurred during the reporting period related to financial services rendered and other revenue generating activities.

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