FAIR VALUE MEASUREMENTS & DERIVATIVE INSTRUMENTS (Financial Impact of Derivative Energy Contracts) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
Mar. 31, 2014	Mar. 31, 2013
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized Net Gain (Loss) Recorded to Regulatory Assets/Liabilities	$ 4	$ 9
TUCSON ELECTRIC POWER COMPANY
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized Net Gain (Loss) Recorded to Regulatory Assets/Liabilities	$ 1	$ 2

Derivative Instruments Increase Decrease to Regulatory Assets Liabilities

No definition available.

Name:	uns_DerivativeInstrumentsIncreaseDecreaseToRegulatoryAssetsLiabilities
Namespace Prefix:	uns_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration