Utility Plant and Jointly-Owned Facilities - Schedule of Asset Retirement Obligations (Detail) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Public Utility, Property, Plant and Equipment [Line Items]
Liabilities Incurred	$ 789,000	$ 7,638,000	$ (1,872,000)
UNS Energy [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Beginning Balance	13,000,000	4,000,000
Liabilities Incurred		1,000,000
Liabilities Settled
Accretion Expense	1,000,000
Revision to Estimated Cash Flows		8,000,000
Ending Balance	$ 14,000,000	$ 13,000,000