Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Segment Reporting Information [Line Items]
Operating revenues	$ 1,044,300	$ 992,287	$ 3,128,186	$ 2,937,840
Net income	166,266	192,443	380,170	381,135
Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	898,878	871,038	2,368,256	2,268,103
Net income	158,498	181,743	322,569	331,883
Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	137,352	113,211	730,644	643,687
Net income	3,555	6,366	46,141	40,205
All Other
Segment Reporting Information [Line Items]
Operating revenues	8,140	8,082	29,576	26,303
Net income	4,213	4,334	11,460	9,047
Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	1,044,300	992,287	3,128,186	2,937,840
Operating Segments | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	898,811	870,975	2,368,041	2,267,905
Operating Segments | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	137,349	113,230	730,569	643,632
Operating Segments | All Other
Segment Reporting Information [Line Items]
Operating revenues	8,140	8,082	29,576	26,303
Intersegment Eliminations
Segment Reporting Information [Line Items]
Operating revenues	(70)	(44)	(290)	(253)
Intersegment Eliminations | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	67	63	215	198
Intersegment Eliminations | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	3	(19)	75	55
Intersegment Eliminations | All Other
Segment Reporting Information [Line Items]
Operating revenues	$ 0	$ 0	$ 0	$ 0