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Accounting for Derivative Instruments and Hedging Activities - Fair Values of Derivative Assets and Liabilities (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
UNS Energy [Member]
Derivatives, Fair Value [Line Items]
Assets	$ 7	$ 14
Liabilities	(15)	(43)
Net Assets (Liabilities)	(8)	(29)
Tucson Electric Power Company [Member]
Derivatives, Fair Value [Line Items]
Assets	4	3
Liabilities	(4)	(9)
Net Assets (Liabilities)		$ (6)

Fair value of derivative asset, presented on a gross basis even when the derivative instrument is subject to master netting arrangements and qualifies for net presentation in the statement of financial position.

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- Definition

Fair value of derivative liability, presented on a gross basis even when the derivative instrument is subject to master netting arrangements and qualifies for net presentation in the statement of financial position.

+ References+ Details