Business Segments (Details) (USD $)	3 Months Ended								12 Months Ended
	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
Financial data by business segment
Operating revenues	$ 693,122,000	$ 1,109,475,000	$ 878,576,000	$ 620,631,000	$ 667,892,000	$ 1,124,841,000	$ 799,799,000	$ 648,847,000	$ 3,301,804,000	$ 3,241,379,000	$ 3,189,199,000
Fuel and purchased power									994,790,000	1,009,464,000	1,046,815,000
Other operating expenses									1,051,000,000	1,058,000,000	1,013,000,000
Operating margin									1,256,000,000	1,174,000,000	1,129,000,000
Depreciation and amortization									404,336,000	427,054,000	414,479,000
Interest expense									199,645,000	223,637,000	227,695,000
Other expense (income)									(4,200,000)	(16,367,000)	(18,532,000)
INCOME FROM CONTINUING OPERATIONS BEFORE INCOME TAXES									656,310,000	539,238,000	505,720,000
Income Taxes	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
INCOME FROM CONTINUING OPERATIONS	34,905,000	252,874,000	130,930,000	284,000	19,544,000	253,273,000	93,185,000	(10,368,000)	418,993,000	355,634,000	344,851,000
Income (loss) from discontinued operations - net of income tax expense (benefit)									(5,829,000)	11,306,000	25,358,000
Income tax expense (benefit) on discontinued operations									(3,813,000)	7,418,000	16,260,000
NET INCOME									413,164,000	366,940,000	370,209,000
Less: Net income attributable to noncontrolling interests									31,622,000	27,467,000	20,156,000
NET INCOME ATTRIBUTABLE TO COMMON SHAREHOLDERS	22,631,000	244,823,000	122,345,000	(8,257,000)	12,564,000	255,359,000	86,685,000	(15,135,000)	381,542,000	339,473,000	350,053,000
Total assets	13,379,615,000				13,111,018,000				13,379,615,000	13,111,018,000	12,393,000,000
Capital expenditures									836,000,000	885,000,000	670,000,000
Regulated electricity segment
Financial data by business segment
Operating revenues									3,294,000,000	3,237,000,000	3,181,000,000
Fuel and purchased power									995,000,000	1,009,000,000	1,047,000,000
Other operating expenses									1,047,000,000	1,055,000,000	1,009,000,000
Operating margin									1,252,000,000	1,173,000,000	1,125,000,000
Depreciation and amortization									404,000,000	427,000,000	415,000,000
Interest expense									200,000,000	224,000,000	226,000,000
Other expense (income)									(9,000,000)	(19,000,000)	(22,000,000)
INCOME FROM CONTINUING OPERATIONS BEFORE INCOME TAXES									657,000,000	541,000,000	506,000,000
Income Taxes									238,000,000	184,000,000	161,000,000
INCOME FROM CONTINUING OPERATIONS									419,000,000	357,000,000	345,000,000
NET INCOME									419,000,000	357,000,000	345,000,000
Less: Net income attributable to noncontrolling interests									31,000,000	28,000,000	20,000,000
NET INCOME ATTRIBUTABLE TO COMMON SHAREHOLDERS									388,000,000	329,000,000	325,000,000
Total assets	13,347,000,000				13,068,000,000				13,347,000,000	13,068,000,000	12,285,000,000
Capital expenditures									836,000,000	885,000,000	666,000,000
All other
Financial data by business segment
Operating revenues									8,000,000	4,000,000	8,000,000
Other operating expenses									4,000,000	3,000,000	4,000,000
Operating margin									4,000,000	1,000,000	4,000,000
Interest expense											2,000,000
Other expense (income)									5,000,000	3,000,000	2,000,000
INCOME FROM CONTINUING OPERATIONS BEFORE INCOME TAXES									(1,000,000)	(2,000,000)
Income Taxes									(1,000,000)	(1,000,000)
INCOME FROM CONTINUING OPERATIONS										(1,000,000)
Income (loss) from discontinued operations - net of income tax expense (benefit)									(6,000,000)	11,000,000	25,000,000
NET INCOME									(6,000,000)	10,000,000	25,000,000
NET INCOME ATTRIBUTABLE TO COMMON SHAREHOLDERS									(6,000,000)	10,000,000	25,000,000
Total assets	33,000,000				43,000,000				33,000,000	43,000,000	108,000,000
Capital expenditures											$ 4,000,000