Segment Information (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011
Segment Reporting Information [Line Items]
Equity investments in unconsolidated subsidiaries	$ 100,300,000		$ 100,300,000		$ 92,700,000
Operating revenues from external customers	2,274,668,000	2,438,222,000	4,852,747,000	5,254,762,000
Intersegment revenues	0	0	0	0
Total operating revenues	2,274,668,000	2,438,222,000	4,852,747,000	5,254,762,000
Income (loss) from continuing operations	183,075,000	158,671,000	366,844,000	362,138,000
Regulated Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues from external customers	2,036,829,000	2,128,397,000	3,973,611,000	4,158,369,000
Intersegment revenues	297,000	356,000	599,000	695,000
Total operating revenues	2,037,126,000	2,128,753,000	3,974,210,000	4,159,064,000
Income (loss) from continuing operations	190,151,000	162,482,000	333,372,000	317,119,000
Regulated Natural Gas [Member]
Segment Reporting Information [Line Items]
Equity investments in unconsolidated subsidiaries	100,300,000		100,300,000		92,700,000
Operating revenues from external customers	221,313,000	291,538,000	842,348,000	1,056,887,000
Intersegment revenues	219,000	597,000	718,000	1,396,000
Total operating revenues	221,532,000	292,135,000	843,066,000	1,058,283,000
Income (loss) from continuing operations	6,190,000	6,596,000	56,392,000	65,193,000
All Other [Member]
Segment Reporting Information [Line Items]
Operating revenues from external customers	16,526,000	18,287,000	36,788,000	39,506,000
Intersegment revenues	0	0	0	0
Total operating revenues	16,526,000	18,287,000	36,788,000	39,506,000
Income (loss) from continuing operations	(13,266,000)	(10,407,000)	(22,920,000)	(20,174,000)
Reconciling Eliminations [Member]
Segment Reporting Information [Line Items]
Operating revenues from external customers	0	0	0	0
Intersegment revenues	(516,000)	(953,000)	(1,317,000)	(2,091,000)
Total operating revenues	(516,000)	(953,000)	(1,317,000)	(2,091,000)
Income (loss) from continuing operations	$ 0	$ 0	$ 0	$ 0