Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Segment Reporting Information [Line Items]
Operating revenues from external customers	$ 869,501	$ 933,100	$ 1,945,553	$ 2,077,547
Intersegment revenues	0	0	0	0
Total operating revenues	869,501	933,100	1,945,553	2,077,547
Net income	95,407	71,018	188,692	167,648
Regulated Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues from external customers	714,651	749,791	1,396,930	1,453,944
Intersegment revenues	43	70	135	184
Total operating revenues	714,694	749,861	1,397,065	1,454,128
Net income	88,807	66,801	150,140	132,516
Regulated Natural Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues from external customers	147,398	174,730	530,402	602,921
Intersegment revenues	19	173	74	219
Total operating revenues	147,417	174,903	530,476	603,140
Net income	5,529	2,661	33,839	31,539
All Other [Member]
Segment Reporting Information [Line Items]
Operating revenues from external customers	7,452	8,579	18,221	20,682
Intersegment revenues	0	0	0	0
Total operating revenues	7,452	8,579	18,221	20,682
Net income	1,071	1,556	4,713	3,593
Reconciling Eliminations [Member]
Segment Reporting Information [Line Items]
Operating revenues from external customers	0	0	0	0
Intersegment revenues	(62)	(243)	(209)	(403)
Total operating revenues	(62)	(243)	(209)	(403)
Net income	$ 0	$ 0	$ 0	$ 0