Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Segment Reporting Information [Line Items]
Operating revenues	$ 966,429	$ 869,501	$ 2,083,886	$ 1,945,553
Net income	97,299	95,407	213,904	188,692
Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	747,882	714,651	1,469,230	1,396,930
Net income	85,603	88,807	164,071	150,140
Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	209,296	147,398	593,220	530,402
Net income	8,786	5,529	42,585	33,839
All Other
Segment Reporting Information [Line Items]
Operating revenues	9,251	7,452	21,436	18,221
Net income	2,910	1,071	7,248	4,713
Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	966,429	869,501	2,083,886	1,945,553
Operating Segments | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	747,943	714,694	1,469,379	1,397,065
Operating Segments | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	209,321	147,417	593,292	530,476
Operating Segments | All Other
Segment Reporting Information [Line Items]
Operating revenues	9,251	7,452	21,436	18,221
Intersegment Eliminations
Segment Reporting Information [Line Items]
Operating revenues	(86)	(62)	(221)	(209)
Intersegment Eliminations | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	61	43	149	135
Intersegment Eliminations | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	25	19	72	74
Intersegment Eliminations | All Other
Segment Reporting Information [Line Items]
Operating revenues	$ 0	$ 0	$ 0	$ 0