Income Taxes (Details 3) (USD $)	1 Months Ended	3 Months Ended					12 Months Ended
	Feb. 28, 2011 Y	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Comparison of pretax income from continuing operations at the federal income tax rate to income tax expense - continuing operations
Federal income tax rate (as a percent)							35.00%	35.00%	35.00%
Federal income tax expense at 35% statutory rate							$ 188,733,000	$ 177,002,000	$ 138,110,000
Increases (reductions) in tax expense resulting from:
State income tax net of federal income tax benefit							19,594,000	17,485,000	15,068,000
Credits and favorable adjustments related to prior years resolved in current year								(17,300,000)
Medicare Subsidy Part-D							823,000	1,311,000	(2,095,000)
Allowance for equity funds used during construction							(6,881,000)	(6,563,000)	(4,265,000)
Palo Verde VIE noncontrolling interest							(9,636,000)	(7,057,000)	(6,723,000)
Other							(9,029,000)	(4,009,000)	(1,544,000)
Income tax expense - continuing operations		7,375,000	131,416,000	50,818,000	(6,005,000)		183,604,000	160,869,000	138,551,000
Net deferred income tax liability recognized on the Consolidated Balance Sheets
Current asset		130,571,000					130,571,000	124,897,000
Long-term liability		(1,925,388,000)					(1,925,388,000)	(1,863,861,000)
Deferred income taxes - net		(1,794,817,000)					(1,794,817,000)	(1,738,964,000)
Income Taxes, additional disclosures
Phase-in period of corporate income tax rate reductions beginning in 2014 (in years)	4
Decrease in deferred income tax liabilities					(62,000,000)	42,000,000
Change in regulatory liabilities					$ 62,000,000		$ 37,009,000	$ 56,801,000	$ 82,650,000