Segments and Related Information (Details) (USD $)	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Dec. 31, 2011
Segment Reporting Information [Line Items]
Equity investments in unconsolidated subsidiaries	$ 92,300,000		$ 92,700,000
Operating revenues from external customers	2,578,079,000	2,816,540,000
Intersegment revenues	0	0
Total operating revenues	2,578,079,000	2,816,540,000
Income (loss) from continuing operations	183,769,000	203,467,000
Regulated Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues from external customers	1,936,782,000	2,029,972,000
Intersegment revenues	302,000	339,000
Total operating revenues	1,937,084,000	2,030,311,000
Income (loss) from continuing operations	143,221,000	154,637,000
Regulated Natural Gas [Member]
Segment Reporting Information [Line Items]
Equity investments in unconsolidated subsidiaries	92,300,000		92,700,000
Operating revenues from external customers	621,035,000	765,349,000
Intersegment revenues	499,000	799,000
Total operating revenues	621,534,000	766,148,000
Income (loss) from continuing operations	50,202,000	58,597,000
All Other [Member]
Segment Reporting Information [Line Items]
Operating revenues from external customers	20,262,000	21,219,000
Intersegment revenues	0	0
Total operating revenues	20,262,000	21,219,000
Income (loss) from continuing operations	(9,654,000)	(9,767,000)
Reconciling Eliminations [Member]
Segment Reporting Information [Line Items]
Operating revenues from external customers	0	0
Intersegment revenues	(801,000)	(1,138,000)
Total operating revenues	(801,000)	(1,138,000)
Income (loss) from continuing operations	$ 0	$ 0