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Fair Value Measurements - Quantitative Information Regarding Unobservable Inputs (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Fair Value Inputs Assets Liabilities Quantitative Information [Line Items]
Forward contract assets	$ 7	$ 14
Tucson Electric Power Company [Member]
Fair Value Inputs Assets Liabilities Quantitative Information [Line Items]
Forward contract assets	3	3
Tucson Electric Power Company [Member] \| Forward Contracts [Member]
Fair Value Inputs Assets Liabilities Quantitative Information [Line Items]
Forward contract assets	1
Forward contract liabilities	$ 2

Fair values as of the balance sheet date of all assets resulting from contracts that meet the criteria of being accounted for as derivative instruments, net of the effects of master netting arrangements.

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- Definition

Derivative instrument obligations meeting the definition of a liability which the Company is a party to as of the balance sheet date.

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