Business Segments (Details) (USD $)	3 Months Ended				12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Financial data by business segment
Operating revenues	$ 667,892,000	$ 1,124,841,000	$ 799,799,000	$ 648,847,000	$ 3,241,379,000	$ 3,189,199,000	$ 3,153,656,000
Fuel and purchased power costs					1,009,464,000	1,046,815,000	1,178,620,000
Other operating expenses					1,058,000,000	1,013,000,000	952,000,000
Operating margin					1,174,000,000	1,129,000,000	1,023,000,000
Depreciation and amortization					427,054,000	414,479,000	407,354,000
Interest expense					223,637,000	227,695,000	227,387,000
Other expense (income)					(16,367,000)	(18,532,000)	(5,858,000)
INCOME FROM CONTINUING OPERATIONS BEFORE INCOME TAXES					539,238,000	505,720,000	394,599,000
Income Taxes	7,375,000	131,416,000	50,818,000	(6,005,000)	183,604,000	160,869,000	138,551,000
INCOME FROM CONTINUING OPERATIONS	19,544,000	253,273,000	93,185,000	(10,368,000)	355,634,000	344,851,000	256,048,000
Income (loss) from discontinued operations - net of income tax expense (benefit)					11,306,000	25,358,000	(183,284,000)
Income tax expense (benefit) on discontinued operations					7,418,000	16,260,000	(109,641,000)
NET INCOME					366,940,000	370,209,000	72,764,000
Net income attributable to noncontrolling interests					27,467,000	20,156,000	4,434,000
Net income attributable to common shareholders	12,564,000	255,359,000	86,685,000	(15,135,000)	339,473,000	350,053,000	68,330,000
Total assets	13,111,018,000				13,111,018,000	12,392,998,000	12,035,000,000
Capital expenditures					885,000,000	670,000,000	745,000,000
Regulated electricity segment
Financial data by business segment
Operating revenues					3,237,000,000	3,181,000,000	3,149,000,000
Fuel and purchased power costs					1,009,000,000	1,047,000,000	1,179,000,000
Other operating expenses					1,055,000,000	1,009,000,000	948,000,000
Operating margin					1,173,000,000	1,125,000,000	1,022,000,000
Depreciation and amortization					427,000,000	415,000,000	407,000,000
Interest expense					224,000,000	226,000,000	226,000,000
Other expense (income)					(19,000,000)	(22,000,000)	(16,000,000)
INCOME FROM CONTINUING OPERATIONS BEFORE INCOME TAXES					541,000,000	506,000,000	405,000,000
Income Taxes					184,000,000	161,000,000	143,000,000
INCOME FROM CONTINUING OPERATIONS					357,000,000	345,000,000	262,000,000
NET INCOME					357,000,000	345,000,000	262,000,000
Net income attributable to noncontrolling interests					28,000,000	20,000,000	19,000,000
Net income attributable to common shareholders					329,000,000	325,000,000	243,000,000
Total assets	13,068,000,000				13,068,000,000	12,285,000,000	11,740,000,000
Capital expenditures					885,000,000	666,000,000	732,000,000
All other
Financial data by business segment
Operating revenues					4,000,000	8,000,000	5,000,000
Other operating expenses					3,000,000	4,000,000	4,000,000
Operating margin					1,000,000	4,000,000	1,000,000
Interest expense						2,000,000	1,000,000
Other expense (income)					3,000,000	2,000,000	10,000,000
INCOME FROM CONTINUING OPERATIONS BEFORE INCOME TAXES					(2,000,000)		(10,000,000)
Income Taxes					(1,000,000)		(4,000,000)
INCOME FROM CONTINUING OPERATIONS					(1,000,000)		(6,000,000)
Income (loss) from discontinued operations - net of income tax expense (benefit)					11,000,000	25,000,000	(183,000,000)
NET INCOME					10,000,000	25,000,000	(189,000,000)
Net income attributable to noncontrolling interests							(14,000,000)
Net income attributable to common shareholders					10,000,000	25,000,000	(175,000,000)
Total assets	43,000,000				43,000,000	108,000,000	295,000,000
Capital expenditures						$ 4,000,000	$ 13,000,000