Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Segment Reporting Information [Line Items]
Operating revenues from external customers	$ 992,287	$ 1,063,934	$ 2,937,840	$ 3,141,481
Intersegment revenues	0	0	0	0
Total operating revenues	992,287	1,063,934	2,937,840	3,141,481
Net income	192,443	140,718	381,135	308,366
Regulated Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues from external customers	870,975	933,490	2,267,905	2,387,434
Intersegment revenues	63	53	198	237
Total operating revenues	871,038	933,543	2,268,103	2,387,671
Net income	181,743	143,702	331,883	276,218
Regulated Natural Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues from external customers	113,230	122,672	643,632	725,593
Intersegment revenues	(19)	(18)	55	201
Total operating revenues	113,211	122,654	643,687	725,794
Net income	6,366	(4,219)	40,205	27,320
All Other [Member]
Segment Reporting Information [Line Items]
Operating revenues from external customers	8,082	7,772	26,303	28,454
Intersegment revenues	0	0	0	0
Total operating revenues	8,082	7,772	26,303	28,454
Net income	4,334	1,235	9,047	4,828
Reconciling Eliminations [Member]
Segment Reporting Information [Line Items]
Operating revenues from external customers	0	0	0	0
Intersegment revenues	(44)	(35)	(253)	(438)
Total operating revenues	(44)	(35)	(253)	(438)
Net income	$ 0	$ 0	$ 0	$ 0