INCOME TAXES (Additional Information) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating Loss Carryforwards [Line Items]
Increase in regulated liabilities due to change in tax rate	$ 9,000,000
Impact of AMT Sequestration	2,000,000	$ (2,000,000)	$ 0
Valuation allowance	0	0
Unrecognized tax benefits, if recognized, decrease in income tax expense (benefit)	1,000,000	1,000,000
Interest expense related to uncertain tax position	0	0
Interest payable	0	0
Penalties accrued	0	0
Decrease in uncertain tax position obligations	17,000,000
Federal tax return	8,000,000	13,000,000	$ 0
UNS Energy
Operating Loss Carryforwards [Line Items]
Federal tax return	14,000,000	$ 0
Internal Revenue Service (IRS)
Operating Loss Carryforwards [Line Items]
Tax credits	$ 7,000,000