Accounting for Derivative Instruments and Hedging Activities - Sum of the Fair Value of all Derivative Instruments Under Contracts with Credit Risk-Related Contingent Features (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012
UNS Energy [Member]
Derivative [Line Items]
Net Liability Position	$ 36
LOCs	1
Additional Collateral to Post if Contingent Features Triggered	36
Tucson Electric Power Company [Member]
Derivative [Line Items]
Net Liability Position	10
LOCs	1
Additional Collateral to Post if Contingent Features Triggered	$ 10