Income Taxes (Details 3) (USD $)	0 Months Ended	3 Months Ended								12 Months Ended
	Feb. 17, 2011	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
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										$ 229,709,000	$ 188,733,000	$ 177,002,000
Increases (reductions) in tax expense resulting from:
State income tax net of federal income tax benefit										23,819,000	19,594,000	17,485,000
Credits and favorable adjustments related to prior years resolved in current year												(17,300,000)
Medicare Subsidy Part-D										483,000	823,000	1,311,000
Allowance for equity funds used during construction										(6,158,000)	(6,881,000)	(6,563,000)
Palo Verde VIE noncontrolling interest										(11,065,000)	(9,636,000)	(7,057,000)
Other										529,000	(9,029,000)	(4,009,000)
Income tax expense - continuing operations		18,157,000	147,116,000	76,689,000	(4,645,000)	7,375,000	131,416,000	50,818,000	(6,005,000)	237,317,000	183,604,000	160,869,000
Net deferred income tax liability recognized on the Consolidated Balance Sheets
Current asset		152,191,000				130,571,000				152,191,000	130,571,000
Long-term liability		(2,151,371,000)				(1,925,388,000)				(2,151,371,000)	(1,925,388,000)
Deferred income taxes - net		(1,999,180,000)				(1,794,817,000)				(1,999,180,000)	(1,794,817,000)
Income Taxes, additional disclosures
Phase-in period of corporate income tax rate reductions beginning in 2014	4 years
Increase in deferred income tax liabilities										(69,000,000)
Change in regulatory liabilities										13,539,000	37,009,000	56,801,000
Percentage of bonus depreciation for federal income tax purposes										50.00%
Federal general business income tax credit carryforwards whose realization will be delayed on recognition of cash benefit		$ 79,000,000								$ 79,000,000