Accounting for Derivative Instruments and Hedging Activities - Realized Losses on Settled Gas Swaps (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
UNS Energy [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Realized Losses on Gas Swaps	$ (22)	$ (19)	$ (23)
Tucson Electric Power Company [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Realized Losses on Gas Swaps	$ (10)	$ (7)	$ (9)